V642 Virginis is a polar spotted, well detached, low mass binary, very likely a Pre-WUMa (T1 ~ 4250 K, ~K6V) eclipsing binary. It was observed on 16, 17 April, 23, 24 May and 7 June 2020 at Dark Sky Observatory in North Carolina with the 0.81-m reflector of Appalachian State University. Four times of minimum light were determined from our present light curves, which include two primary eclipses and two secondary eclipses: HJD Min I = 2458955.57893 ± 0.00039, 2458955.83742 ± 0.00023 and HJD Min II = 2458956.61203 ± 0.000007, 2458955.87052 ± 0.00041. In addition, twelve TESS timings were calculated, and many timings were taken by our co-authors and others. These are included in our 21-year period study. A total of 87 timings were used in our period study. From these timings, we determined both linear and quadratic ephemerides: JD Hel Min I = 2458956.60725(36) d + 0.516644467(65)×E, JD Hel Min I = 2458956.61101(18) d + 0.5166463518(70)×E + 0.0000000000159(5)×E2.
The O-C plots of the ephemerides, however, show an oscillation of residuals which points to the existence of a third body, an M5V dwarf (0.16 Solar Masses), in an eccentric orbit (e = 0.41), with period = 20.07 years. V642 Vir’s odd light curve is deep with “V” shaped eclipses. It reaches maxima just before and following the secondary eclipse, which indicates that it has polar spots similar to UV Leo and the recently published V1023 Per. The presently fixed polar spot region indicates that V642 Vir must have a strong magnetic field, and that it is synchronously rotating. The BVRcIc simultaneous 2016 Wilson-Devinney Program (W-D) solution gives a detached solution (primary and secondary components are under-filling their respective Roche Lobes, with 76% and 78% fill-outs, respectively). The cool spot region, models near the pole of the primary component (centered at 10° colatitude) and is angled toward the secondary component. Its large radius (68°) and T-fact (0.69) also contribute to the conclusion about the strength of the magnetic field. In addition, a 17° Southern hot spot is modeled. The small ΔT in the components (~318 K) and mass ratio near unity (0.9542 ± 0.0005) show that the stars are similar in spectral type (secondary ~K9V). The inclination is high ~ 86.87 ± 0.04°, but there is no time of constant light due to the two stars’ essentially equal radii. The systems distance is ~185 pc as determined from GAIA DR2.