25 Serpentis

This article is about A² Serpentis. For other stars with this Bayer designation, see A Serpentis.

25 Serpentis is a star system in the constellation of Serpens Caput. With an apparent magnitude of 5.37, it is just barely visible to the naked eye. The system is estimated to be some 450 light-years (138 parsecs) based on its parallax.^[1]

A light curve for PT Serpentis, plotted from Hipparcos data^[10]

25 Serpentis

Location of 25 Serpentis (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Serpens
Right ascension	15h 46m 05.63636s[1]
Declination	−01° 48′ 15.0830″[1]
Apparent magnitude (V)	5.37 - 5.40[2]
Characteristics
Spectral type	B8IV/V[3]
U−B color index	−0.40[4]
B−V color index	−0.05[4]
Variable type	SPB[5]
Astrometry
Radial velocity (Rv)	9.50±0.3[6] km/s
Proper motion (μ)	RA: −23.15[1] mas/yr Dec.: −30.50[1] mas/yr
Parallax (π)	7.25±0.31 mas[1]
Distance	450 ± 20 ly (138 ± 6 pc)
Orbit[7]
Period (P)	38.927±0.004
Eccentricity (e)	0.731±0.006
Periastron epoch (T)	2450132.80±0.06
Argument of periastron (ω) (secondary)	201.8±0.8°
Semi-amplitude (K1) (primary)	43.1±0.7 km/s
Semi-amplitude (K2) (secondary)	86±1 km/s
Details[8]
Mass	4.1 M☉
Luminosity	383 L☉
Temperature	13,932 K
Rotational velocity (v sin i)	80 km/s
Other designations
A2 Ser,[9] 25 Ser, PT Ser, HD 140873, HIP 77227, HR 5863, SAO 140740, BD−01°3092
Database references
SIMBAD	data

25 Serpentis is a spectroscopic binary,^[11] 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.^[7] The primary is a slowly pulsating B-type star, which causes the system to vary by 0.03 magnitudes;^[2] for that reason it has been given the variable star designation PT Serpentis.