25 Serpentis

25 Serpentis is a star system in the constellation of Serpens Caput. With an apparent magnitude of 5.4, it is just barely visible to the naked eye. The system is estimated to be some 450 light-years based on its parallax.
File:PTSerLightCurve.png|thumb|left|A light curve for PT Serpentis, plotted from Hipparcos data
25 Serpentis is a spectroscopic binary, meaning that the individual components are too close to be resolved, but periodic Doppler shifts in their spectra indicate orbital motion. The system consists of a hot B-type giant and an A-type main-sequence star. The two stars orbit each other every 38.9 days, and have a very eccentric orbit, with an orbital eccentricity of 0.731. The star was found to be a variable star in 1996, when the Hipparcos data was analyzed. The primary is a slowly pulsating B-type star, which causes the system to vary by 0.03 magnitudes; for that reason it was given the variable star designation PT Serpentis in 1999.