Thank you, vipa. Is Richard Thomas (Sr) still alive? When you look at the still image of Pas de Dix - Part 2, Sobotka & O'Brien are the third couple to the left - or the second couple to your right - depending upon how you view it. She is wearing glittering earrings. Sobotka also does a solo at the end of Pas de Dix - Part 1; she is the soloist on the right.
And I believe we see Ruth Sobotka as Apollo's mother, Leto, in the d'Amboise version of Apollo on DVD. She was Stanley Kubrick's second wife.