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Summary: Mon. Not. R. Astron. Soc. 000, 1{19 (0000) Printed 22 January 2001 (MN L A T E X style le v1.4)
Cyclic and secular variation in the temperatures and radii
of extreme helium stars ?
C. Simon Je ery 1;2 , Rhaana L.C. Starling 1;2 y , Philip W. Hill 2 z , and Don Pollacco 3;2
1 Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
2 Dept of Physics and Astronomy, University of St Andrews, St Andrews, FIFE KY16 9SS, Scotland
3 Dept of Physics, Queens University Belfast, Belfast BT7 1NN, Northern Ireland
Accepted . Received ;
ABSTRACT
The ultraviolet properties of seventeen extreme helium stars have been exam-
ined using 150 IUE spectra. Combining short-wave and long-wave image pairs and
using a grid of hydrogen-de cient model atmospheres and a 2 -minimization proce-
dure, seventy measurements of e ective temperature ( T e ), angular diameters () and
interstellar extinction ( EB V ) were obtained. In most cases, these were in good agree-
ment with previous measurements, but there are some ambiguities in the case of the
hotter stars, where the solutions for T e and EB V become degenerate, and in the
case of the cooler stars with large EB V where the total
ux is no longer dominated
by the ultraviolet.
The behaviour of twelve helium stars was examined over an interval exceeding ten
years. The surfaces of four stars (HD168476, HD160641, BD{9 Æ 4395 and BD{1 Æ 3438)
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