S Orionis: A Mira-type variable with a marked period decrease

Departamento de Física, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain e-mail: pedromer@unex.es

Corresponding author: M. Jurado Vargas, mjv@unex.es

Received: 14 November 2001
Accepted: 30 January 2002

Abstract

We studied the pulsational period of the Mira star S Orionis based on visual observations that cover a total of 71 years. We found that the period decreased markedly from around 445 days to 397 days in approximately 16 years, between JD 2438000 and JD 2444000. The rate of this period variation was of the order of 0.007 day/day, too fast for the usual variations observed in most Mira variables. This result is in good agreement with the theoretical models that suggest a helium-shell flash as the cause of these large-period variations. In particular, the variation of the period and luminosity indicates that this Mira star may now be in an immediate post-primary helium-shell flash state.