The Reeperbahn, where the Indra was situated, was a hundred times less salubrious than our own Soho. Sex, booze, music and pills were the name of the game, and these young "innocent" lads were right in the middle of it all. They worked very long hours in those German clubs and it was the best thing that could have happened to them as far as their music was concerned, improving as they did way beyond my wildest dreams. Their technique, their staying power, the sheer guts and enthusiasm that went into their every performance, the sound that they produced -- all this stretched out and tore at your very being. It was so total in its concept that no one could escape it, least of all one Astrid Kirschner, and Klaus Voorman, her boyfriend at the time. Astrid was a photographer, a very beautiful girl whose family was upper-class German and whose ideas were as way-out as the Beatles themselves. The moment Astrid set eyes on the boys and heard for herself the incredible sounds that they were producing she became totally addicted and made sure that she was introduced to them.

-- from Cynthia's A Twist of Lennon

Cynthia continues with John raving to her in his letters about Astrid. She mentions fearing that John would leave her for Astrid -- only to learn that Astrid instead fell in love with Stuart.

BTW, while one could understand Cynthia being jealous of Astrid, what with all of John's raving about her, Cynthia and Astrid in fact became good friends very quickly.

In her Twist of Lennon, Cynthia goes on quite a bit about Astrid struck her as a beautiful, exotic, unique woman of the future.