COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2411

                                      Konkoly Observatory
                                      Budapest
                                      14 October 1983

                                      HU ISSN 0374-0676


            THE UNIQUE DOUBLE-MODE CEPHEID CO Aur




   Recently, Mantegazza (1983) has analysed the photoelectric observations 
of the irregular variable star CO Aur made by Smak (1964) and
DuPuy and Randall (1974), and has shown that this star is a double-mode, 
short period Cepheid pulsating in the first and second radial
overtones. At the present, notwithstanding several surveys have been
performed, only eleven double-mode Cepheids are known, and all of them
are pulsating in the fundamental and first overtone radial modes.
   In order to confirm that CO Aur is a double-mode Cepheid, we
observed this star in the UBV colours with the photometer attached to
the 102 cm telescope of Merate Observatory, from January to March, 1983


                              [FIGURE 1]


(thirteen nights). The instruments and the observational techniques
are the same as those described by Antonello and Mantegazza (1982);
in particular, in order to check the accuracy of our measurements, two
comparison stars, BD+35d1308 and BD+35d1311, were adopted.
   The observations confirm the previous result, that is there are
two periodicities, P_1 = 1.783 d and P_2 = 1.430 d. The figure shows the
B data prewhitened for the second periodicity and the first order coupling 
terms and phased with P_1, and the B data prewhitened for the
first periodicity and the first order coupling terms and phased with
P_2.
   Full details on the data and a discussion on the importance of
the Fourier analysis of the double-mode Cepheid light curves for the
determination of the pulsating modes in Cepheids will be published 
elsewhere.





             E. ANTONELLO and L. MANTEGAZZA
             Osservatorio Astronomico di Brera
             Via E. Bianchi 46
             22055 Merate, Italy





References:

Antonello, E. and Mantegazza, L.: 1982, Astron. Astrophys. Suppl. 49,
  709. [BIBCODE 1982A&AS...49..709A ]
DuPuy, D.L. and Randall, C.B.: 1974, The Observatory 94, 71. [BIBCODE 1974Obs....94...71D ]
Mantegazza, L.: 1983, Astron. Astrophys. 118, 321. [BIBCODE 1983A&A...118..321M ]
Smak, J.: 1964, Publ. Astron. Soc. Pacific 76, 40. [BIBCODE 1964PASP...76...40S ]