The Postmaster's Daughter w/ Direct link technology (A Mystery Classic)

The other woman in the case is a mere slip of a girl, Doris Martin, the postmaster’s daughter of the title. She lives across the river and she and Grant signal back and forth from home and garden. There is an as yet unacknowledged attraction between them, and the pair were in Grant’s garden the evening of the murder stargazing at Sirius at a time deduced to be not long before the murder took place.