LONG-TERM UBV(RI)(C) MONITORING OF 12 SOUTHERN-HEMISPHERE LONG-PERIODVARIABLES

A program of UBV(RI)(c) photometric observations of twelve southern he misphere 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 s pectral types from the (V-R-c) and (R-c-I-c) colours was also performe d using the method described by Cells (1986b). We therefore present fo r the first time spectral type variations together with visible and co lour light-curves for at least three successive cycles. New photometri c parameters as visible and spectral type extrema are given. Spectral type variations are found in the range 1.3 - 4.5 subclasses. The varia tions during a cycle and from cycle-to-cycle are discussed and short-t erm declines with large amplitude are reported for R Oct and RY Hyi. W e finally estimate the distance of these stars using a (M-V(max), P, S pectral type) relation. It is shown that rather good distances (mean e rror less than 30%) can be determined if the period of the stars is kn own and good UBVRI data are collected at the maximum of luminosity.(1)