COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1239

                                                    Konkoly Observatory
                                                    Budapest
                                                    1977 February 14


A PERIOD-LUMINOSITY RELATION FOR SHORT PERIOD MIRA VARIABLES

The validity of the period-luminosity relation of Mira
type variables, published previously by the present author
(Ferrari, 1973), was limited to stars with periods nearly equal
to, or longer than 160 days. Now, another formula has been 
established for short period variables of the same type. On the
assumption that the latter ones are undergoing overtone pulsations, 
a formal similarity between the period-luminosity relations 
of both groups, and a numerator of about 100 (instead of
200) days for the shorter periods were theoretically reasonable.
The expected slope of the function wanted, the constant part of
denominator, and the zero point - by chance reducing the additional 
constant itself to zero - could be derived from observational 
data (Osvalds & Risley, 1961; Clayton & Feast, 1969).
   Hence, M_m being the visual absolute magnitude of mean
maxima of any Mira type variable with mean period P days, it has
been found that

           M_m = - 100/(P-67);   100d <= p <= 158d.

The upper limit of validity was suggested by a pronounced 
frequency gap, while typical Mira variables with periods shorter
than 100 days are lacking at all, and therefore the extrapolation 
beyond the lower limit by no means could be warranted. But
within the limits specified here, slight changes of the numerator 
might be compensated by suitable adjustments of the constant
part of the denominator and an additional constant restoring
the zero point, without significant deviations from the results
of the preceding two-parametric formula.
   A detailed discussion of the problem will be published
elsewhere.


                         K. FERRARI D'OCCHIEPPO

                    Institut fur Theoretische Astronomie
                          A-1189 Wien / Austria



References:

Clayton, M.L. & Feast, M.W., 1969, Monthly Not.R.A.S. 146, 411 [BIBCODE 1969MNRAS.146..411C ]
Osvalds, V. & Risley, A.M., 1961, Publ. Leander McCormick Obs. vol. XI., part
   XXI
Ferrari d'Occhieppo, K., 1973, Information Bull. Var. Stars No. 768