Long-term UBV (RI)_c monitoring of 12 southern hemisphere Long Period Variables ^*,**

¹ Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden
² GRAAL, Université Montpellier II, F-34095 Montpellier Cedex 05, France
³ ESO, Casilla 19001, Santiago 19, Chile
⁴ ARPEGES, Observatoire de Paris, place J. Janssen, 92195 Meudon Principal Cedex, France
⁵ DAEC, Observatoire de Paris, place J. Janssen, 92195 Meudon Principal Cedex, France

Send offprint request to: P. de Laverny (Uppsala Observatory)

Received: 29 February 1996
Accepted: 9 August 1996

Abstract

A program of photometric observations of twelve southern hemisphere Long Period Variables (LPV or Miras) has been carried out at the European Southern Observatory. Each star was observed on at least 11 occasions up to 29 over a 4-year period. A determination of their spectral types from the ( R_c) and (R_c ) colours was also performed using the method described by Celis (1986b). We therefore present for the first time spectral type variations together with visible and colour light-curves for at least three successive cycles. New photometric parameters as visible and spectral type extrema are given. Spectral type variations are found in the range 1.3-4.5 subclasses. The variations during a cycle and from cycle-to-cycle are discussed and short-term declines with large amplitude are reported for R Oct and RY Hyi. We finally estimate the distance of these stars using a (, P, Spectral type) relation. It is shown that rather good distances (mean error less than 30%) can be determined if the period of the stars is known and good UBVRI data are collected at the maximum of luminosity.