Two things, however, were necessary to accomplish this: firstly, that M. de Lauzun should thoroughly understand that he was beloved, and that he would deign to espouse Mademoiselle's twenty-two millions; and next that King Louis should consent to a marriage, the strangest certainly ever resolved upon. Strange, indeed, that she, the grand-daughter of Henry the Great, Mademoiselle d'Eu, Mademoiselle de Dombes, Mademoiselle d'Orléans, Mademoiselle the King's first cousin, the Mademoiselle destined to the throne, should ask the King's permission to marry a Gascon cadet. Louis, as the sequel to an overture made to him by several nobles collectively, friends of Lauzun, with M. de Montausier at their head, granted his permission. But when the question arose, thanks to the blind vanity of Lauzun, of their union being celebrated at the Louvre and in the face of all France, like an alliance "of crown to crown;" when a feeling which was shared by every member of the royal house was on the point of communicating itself to all the sovereign families of Europe, Louis, with great reason, began to take account of the political interests which this whim of the Princess brought into play, and retracted, as King, the authority which he had given as head of a family. Contemporary writers seem never tired of dwelling upon the manifestations of Mademoiselle's grief, at times as laughable as at others it was touching; receiving the condolence of all the Court as though she had been a lone widow, Madame de Sévigné tells us, and exclaiming excitedly in her despair to every fresh visitor, as she pointed to the vacant place in her bed, "He should be there! he should be there!"
This took place on the 18th of December, 1670. On the 25th of November, 1671, M. de Lauzun was arrested, thrown into the Bastille, and taken thence to Pignerol, where he was subjected to a captivity of ten years. What passed in that interval has proved a great subject of controversy amongst ingenious writers. The most probable explanation seems to be that, notwithstanding the King's refusal, the marriage between Lauzun and Mademoiselle had been accomplished. The evidence of twenty different persons might be cited in support of the fact, but one may suffice. An historian of the last century, M. Anquetil, relates that at the Château d'Eu, in 1774, an apartment was still pointed out which had been occupied by Lauzun, situated above that of the princess, and communicating by a secret staircase with her alcove. At the same period, Anquetil saw at Treport a tall person resembling Mademoiselle not only in her figure, but strikingly like her portraits. She seemed to be about seventy or seventy-five. She was called, throughout that part of the country, the Princess's daughter. She seemed to believe so herself, and was in receipt of a pension of fifteen hundred francs paid punctually, without knowing from what quarter they came. She occupied a handsome house for which she paid no rent, although for it she held no proprietary deed. All this, coupled with the age of the lady, who stated that she was born in 1671, would seem decisive as to the clandestine marriage which probably occasioned the arrest of Lauzun.