Cyclic and secular variation in the temperatures and radii of extreme helium stars

The ultraviolet properties of 17 extreme helium stars have been examined us ing 150 IUE spectra. Combining short-wave and long-wave image pairs and usi ng a grid of hydrogen-deficient model atmospheres and a chi (2) minimizatio n procedure, 70 measurements of effective temperature (T-eff), angular diam eters (theta) and interstellar extinction (EB-V) were obtained. In most cas es, these were in good agreement with previous measurements, but there are some ambiguities in the case of the hotter stars, where the solutions for T -eff and EB-V become degenerate, and in the case of the cooler stars with l arge EB-V, where the total flux is no longer dominated by the ultraviolet. The behaviour of 12 helium stars was examined over an interval exceeding 10 yr. The surfaces of four stars (HD 168476, HD 160641, BD -9 degrees 4395 a nd BD -1 degrees 3438) were found to be heating at rates between 20 and 120 K yr(-1), in remarkable agreement with theoretical predictions. This resul t provides the first direct evidence that extreme helium stars are helium s hell-burning stars of up to similar to0.9 M-. contracting towards the white dwarf sequence. Low-luminosity helium stars do not show a detectable contr action, also in agreement with theory, although one, BD +10 degrees 2179, m ay be expanding. The short-term behaviour of three variable helium stars (PV Tel variables: HD 168476, BD +1 degrees 4381, LS IV -1 degrees2) was examined over a short interval in 1995. All three showed changes in T-eff and theta on periods c onsistent with previous observations. Near-simultaneous radial velocity (v) measurements were used to establish the total change in radius, with some reservations concerning the adopted periods. Subsequently, measurements of the stellar radii and distances could be deri ved. With T-eff and surface gravities established previously, stellar lumin osities and masses were thus obtained directly from observation. In the cas e of HD 168476, the mass is 0.94 +/-0.68 M-.. Assuming a similar gravity fo r LS IV -1 degrees2 based on its neutral helium line profiles, its mass bec omes 0.79 +/-0.46 M-.. The theta amplitude for BD +1 degrees 4381 appears t o be overestimated by the IUE measurements and leads to a nonsensical resul t. These first direct measurements of luminous extreme helium star masses a gree well with previous estimates from stellar structure and pulsation theo ry.