Right on cue, Prof. Haleh Emrani (University of California) published "Like Father, Like Daughter: Late Sasanian Imperial Ideology and the Rise of Bōrān to Power."*
That's Bōrān, as in Queen Bōrān (left), who became the King of Kings.
HOW THE QUEEN BECAME A KING
A little historical background.
Bōrān's father was Khusrau II (590-628 CE), also called 'the Victorious'. In a series of campaigns, he led his armies against the Byzantines, and, for a short time, expanded the limits of the Sassanian Empire to the greatest extent it had ever reached.
He managed, in effect, to destroy the might of Byzantium, first breaking its military power in a long war of attrition and then taking over all its rich Middle Eastern provinces. The king conquered Syria, Palestine, Egypt and Libya.
The conquest of Jerusalem in 614 had more serious consequences than just the destruction of the city (once again!): this time, the relic of the True Cross fell into Persian hands. They carried it off to their capital, Cteisiphon (near modern Baghdad), where it became a further casus bellum.
Khusrau's armies went on to invade Anatolia and in 615 (or possibly 626), their advance guards halted only a mile from the walls of Constantinople. The city was put under siege. The King of Kings had reached the zenith of his power, and it seemed as though the Persians would finally conquer their ancient foes.
Pride Goeth Before a Fall
Khusrau became puffed up with vainglory because of the vast amount of wealth and all kinds of jewels, utensils, equipment, and horses he had accumulated. He had conquered so many of the lands of his enemies. However, he was filled with conceit and boastfulness, and was horribly avaricious....The Persians had found little opposition in their conquests because of internal conflicts within the Byzantine empire. Now, however, a new emperor, Heraclius, struck back. He sailed with an army into the Black Sea while the Persian army was still on the shores of the Bosphorus. Landing in the Caucasus, he marched into Armenia and Azerbaijan striking into the heart of the Sassanian empire. Capturing Ganzak, the most important Zoroastrian fire-temple in Azerbaijan, he defeated the Persian army sent against him.
[He] had in his palace 3,000 women with whom he had sexual relations and thousands of slave girls employed as servants, for making music and singing, and such. He also had 3,000 male servants, 8,500 riding beasts on which he could travel, 760 elephants, and 12,000 mules for conveying his baggage.
In 628, Heraclius was near Cteisiphon where he captured Khusrau's royal residence. This was a palace worthy of a King of Kings. Heraclius looted the astonishing riches that Khusrau had accumulated over the years, to the tune of 600,000,000 silver coins plus 'purses' of treasure worth another 68,571,42, as well as, as al-Tabari helpfully explains, "various kinds of jewels, garments, and the like, whose grand total God alone could enumerate." 
Despite this turning of the tables, Khusrau still refused to make peace. The landed and military aristocracy, though, had had enough of his ruinous taxes and bloody wars. Persian generals revolted and, led by Khusrau's son, Kavad II, the aging King of Kings was seized, tortured, and slain.
To secure his throne, Kavad killed all male descendants of the royal house, including 16 (or 18) of his brothers. A wise precaution but it turned out to be of little use: after a mere eight months, Kavad died, either of the plague which was spreading during his short reign, or by poison: So a man may reign for seven months, and in the eighth he finds that his crown is made of the camphor with which the dead are anointed [Shahnameh].
His son, Ardashir III, seven years old, succeeded to the throne to be a short-lived puppet of the nobility. Within a few months, the young king was murdered by an army commander, Shahrbaraz, who was not of royal blood. The regicide thus committed the additional enormity of usurping the throne when he was not from the Sassanid house. He reigned for 40 days before being killed in turn by disgruntled nobles. His body was tied to chains and dragged through the streets of the capital.
9 June 629 CE Queen Bōrān, eldest daughter of Khusrau II, was crowned 'Great King'
In the long history of the Sassanian empire, no woman had ever been sole ruler of Persia (though queens had acted as regents).
The Persian conception of royalty was strictly masculine. The king was the chief of the priests and of the Zoroastrian church, which did not have any female clergy. His physical perfection was necessary for his high function (and that must have included his sexual organs): any mutilation was sufficient reason for excluding a prince from the succession. The king's religious, military and political authority was based on the god-given 'Divine Glory' (khvarrah) which, it goes without saying, descended in the male line from the mythical first king, Sāsān. The polygamous family of the king and his rights over the harem and his relatives do not seem to support any independent source of power for queens or princesses. In fact, like any other women, they were presumably always under the authority of a guardian -- whether father, husband, son, or other male relative.
So what happened in on 9 June 629 CE to change four hundred years of royal tradition and raise a woman to the throne?
The nobles and priests, who were the pillars of the Sassanian monarchy, would not accept as king anyone who was not a member of the royal family. So, in the name of defending the house of Sāsān, they killed Persia's best general and put a princess on the throne. In short, a woman from the royal house was considered a more legitimate king than a general who, although of noble birth, was not descended from the blood of Sāsān.
Al-Tabari writes that the nobles were compelled to accept Bōrān as king because of the lack of legitimate male candidates. Certainly, Kavad II had tried to exterminate all possible competitors. But, with his father having had 3,000 sex objects in his harem, there's only so many descendants you can murder in eight months. Just as they later found, hidden here and there, some minor male relatives who had escaped the slaughter and raised them, briefly, to the throne, the lack of males could not be the whole reason for their unexpected choice. Rather, we may imagine that the coronation of Bōrān was a political decision made by the council of nobles. It's certainly not beyond possibility that she herself was involved in the plot against the usurping general.
In any case, her first challenge was to prove she was the rightful holder of the khvarrah. And so she invoked her father's legacy by imitating his crown (left) and coins. Remember, every Sassanian ruler had his own individual crown (that's how we can tell them apart even when their names are not given). Khusrau II had worn the symbol of Wahrām, the god of war and victory, spreading his wings. Bōrān's adoption of this deity can be seen as a symbolic tie to her father -- who was, and in memory remained -- a great warrior-king.
Besides her crown, her coins displayed (upper left, near her chin) the sun, moon, and stars, as well as the crescent moon and stars in the margin of her coins (middle left; below left). The reverses were exactly like those on her father's coins, with a Zoroastrian fire altar attended by two standing figures, surrounded by astral signs in the margins (right). The presence of these signs on her crown and coins backs up her claim to rule with divine sanction. In case of any lingering doubt, a gold coin reads, "Bōrān, restorer of the race of the gods".
Her appearance, too, conformed to kingly standards. In a now-lost book depicting Sassanian kings, Bōrān was shown seated on a throne wearing male royal dress, including a green tunic over sky-blue trousers -- strikingly different from the very long dresses that covered the legs and feet of royal women. Accessories included a sky-blue crown and a battle-axe held in her hands.
So Bōrān was no longer seen as a princess but as a monarch in her own right, dressed in proper kingly attire, and confirmed by the legends on her coins and her crown. These symbols rendered her gender irrelevant. She was, in fact, the king!
xwadāy zan-ēw bawēd "A woman will be king"
The main tasks of a legitimate king was to protect the people of Persia and maintain its religious laws. How did Bōrān handle the duties that justified her khvarrah?
First, she concluded a treaty of peace with the Byzantines. A preliminary agreement had already been reached between Heraclius and General Shahrbaraz or perhaps even earlier with Kavad II. Persia had lost the long war and they knew it. Except for the important city of Nisibis (modern Nusaybin, near the Turkish-Syrian border), all of her father's conquests were returned to Byzantium. Bōrān swallowed the bitter pill and signed.
A prominent part of the peace negotiations was about the True Cross. Although al-Tabari  tells us that Bōrān restored it to Heraclius, it was probably part of an earlier deal: the Cross was back in Jerusalem by spring 630 or, at the latest, Easter 631: in any event, Heraclius and his second wife Martina secured its triumphant return to the Church of the Holy Sepulcher.
Bōrān's reign is said to have been marked by benevolence. She behaved kindly and justly to her subjects. She remitted for the people the arrears of land tax due. Nor did she lack energy: she ordered the rebuilding of stone bridges and bridges made of boats in order to improve the catastrophic economic situation in the empire. She fulfilled her religious duties and established at least one new fire-temple near the capital city.
The extraordinary thing is that all our sources seem to emphasize the positive qualities of this female king. None object to her because of her sex, nor do they reflect any controversies in her rise to the throne.
Too Good To Last
Her coins, of which she minted many, give us this progress
- Year 1 (top left) began just after the Sassanian New Year on 17 June 629 - 16 June 630.
- Year 2 (middle left) ran from 17 June 630 - 16 June 631.
- Year 3 (lower left) would have started on 17 June 631 ... and then she died.
THAT'S NOT THE END OF THE STORY
Bōrān may (or may not) have been followed on the throne by a man called Jushnas Dih, who (if he existed at all) lasted less than a month; then got the chop.
The next certain ruler was Bōrān's own younger sister, Azarmidokht (meaning 'daughter of the honoured one').
Yes, another woman.
Nobles and priests apparently thought the experiment with a female monarch was worth repeating. When she assumed the royal power, she proclaimed in a tougher tone than ever used by her sister:
Our way of conduct will be that of our father, Khusrau, the victorious one, and if anyone rebels against us, we will shed his blood.
On the left is one of her rare coins.
Do not adjust your computer: Azarmidokht is not a bearded lady; at least I don't think she is** -- though minted with her name, the coin depicts a bearded man, presumably her father (compare his crown with Khusrau's crown above).
Her reign is portrayed favourably, shedding light on all because of her remarkable beauty and intelligence. She was 'vigorous' and, in the little time granted her, built a fire-temple.
The only other story told about her is decidedly peculiar. One of the great men of Persia asked for her hand in marriage:
She wrote back, "Marriage to a queen is not permissible. I realize full well that your intention in what you are proposing is to satisfy your own [sexual] needs and lust with me. So come to me on such-and-such a night." Azarmidokht ordered the commander of her guard to lie in wait for him on the night they had agreed to meet together and then kill him. The commander of her guard carried out her orders...; and at her command, the corpse was dragged out by the feet and thrown down by the open space before the palace.The trap arranged by Azarmidokht suggests a rather strong refusal to submit to any man. She would have no husband and no guardian, certainly not one who was not of royal blood. She clearly planned to rule alone. The concept of the royal and divine khvarrah, restricted to the descendants of Sāsān, was enough to block any attempt at usurpation of power. The fate of her would-be 'lover' was thus a political act.
A clever plan. But perhaps unwise.
The great man's son, Rustam, came with a mighty army to avenge his father. After just six months on the throne, Azarmidokht was raped, blinded, and then killed.
Five years later, in 636 CE, this Rustam was the Persian general who lost the battle of Qādisiyyah to the newly united Muslim Arabs. Trying to flee the battlefield, he was caught and beheaded by an Arab soldier.
Who says there is no justice?
The collapse of the Sassanian Empire followed hot on his heels.
* H. Emrani, "Like Father, Like Daughter...." at e-Sasanika 9 (2009). I have also made much use of A. Panaino, "Woman and Kingship..." in J.Wiesehöfer, P.Huyse (eds.) Eran ud Aneran (2006) 221-239.
** Prof. Emrani, however, does think that the beard is meant to emphasize that Azamidokht was not a woman but a king (p. 9); that would be rather, I imagine, in the mode of bearded Hatshepsut.
All coins from Coins of the Sasanian Empire
Drawing comparing crowns of Khusrau II and Bōrān, from Emrani, at e-Sasanika 9 (2009), p. 11.