A Hint of Starlight by Lynne Connolly

In a starlit garden, Lady Damaris Dersingham meets a man who makes her heart beat faster. But the next day she is witness to the greatest humiliation of her life, when she is rejected from the Royal Society. That was all Damaris had ever wanted.
The second Dersingham triplet, Damaris is passionate about astronomy. But that’s not good enough for the Royal Society who rejects her—because she’s a woman. And the man from that starlit garden is none other than Logan, the Duke of Glenbreck, a member of the committee that rejected her.
Discovering the transit of Venus seems ironic to Logan, the Duke of Glenbreck, once he discovers his greatest rival is the most fascinating woman he’s ever met. Damaris deserves better, and Logan will ensure she gets it. The desire he feels for her has nothing to do with it. Truly it doesn’t. When her work is stolen, suspicions arise between them. Did Logan show her work to somebody else? Can she really be in love with him?
Although he has fought his desire for the lovely Damaris, Logan can’t deny it any longer. And when the thief confronts Logan with a stark choice – his work or Damaris’s life – he has to trust his heart rather than his head.