WX Her, a Mira with decreasing period.

Svensk Amator Astronomisk Forening, variabelsektionen (Sweden) (SAAF)
Tue, 04/12/2022 - 15:40

WX Her is one of the stars in SAAF:s (Association of Swedish AmateurAstronomers) observation program "50 forgotten miras" that has run for 10 years. This star seems to have a steady decreasing period over the last 100 years. From DASCH, NSVS and our data I have got 51 maxima from 1927 to 2022. In DASCH there are even older data, (from the 1890's), but to sparse to compile times of maximum.

From the observations I computed preliminary periods over some timespans with Peranso and got this periods:
JD 2411000 to 2425000: 186.6
JD 2425000 to 2435000: 185.1
JD 2440000 to 2448000: 183.0
JD 2456000 to 2459000: 181.0

With these periods I constructed a O-C diagram over the 51 maxima. The diagram has a parabolic look, a parabolic O-C indicate a steady period change. The parabolic coefficient for WX Her is -0.023 which correspond to a decresing period of 0.046 days/cycle or about 8 days over 100 years. The current period is about 180 days.

A residual O-C diagram with the parabolic component removed shows a pretty normal O-C for a mira with random scatter. Unfortunally there are some large gaps in the series of maxima where I have not found any observations which makes the history for the star somewhat unclear. But I think the star is so interesting that i want to draw attention to it.

Some other stars with similar behaviour is SU Vir, Z Vir and DO Her.

Thomas Karlsson