COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 1154

                                Konkoly Observatory
                                Budapest
                                1976 July 20

       ON THE PERIOD OF THE ECLIPSING BINARY UX Mon


   The eclipsing binary UX Mon (BD -7d2291) was observed 
photoelectrically by the writer (Scaltriti 1973) during the winter
1972-73 with a yellow filter at the Astronomical Observatory of
Torino and at McDonald Observatory with a V filter. The system
is very interesting owing to the long-time changing features
shown by the lightcurve, probably due to an intrinsic variability
of the hotter component together with masses of gas flowing 
between the components whose evidence was pointed out by Struve
(1947).
   A study of the period was made by Whitney (1956) who 
concluded that the period was not constant.
   From my observations, I obtained the following epoch of
minimum light: Min_I(Hel.) = 2441719.203+ 0.006.
   Most of the available epochs of primary minimum of UX Mon
are listed by Whitney (1956). We may add the following ones, I
could find in the literature:
2427536.506 (pg) (Gaposchkin 1953)
  35129.722 (ph) (Lynds 1956)
  36227.992 (ph) (Whitney 1959)
  36635.383 (ph) (Whitney 1959)
  41719.203 (ph) (Present work)

    Figure 1 represents the (O-C)'s of all the known epochs
with respect to the ephemeris:

         MinI (Hel.) = 2433346.566 + 5.904574d E

    Note the extremely high value of the (O-C)'s for the epoch
given by Soloviev (1943).

                        [FIGURE 1]

   Figure 1. (O-C)'s of the epochs of minimum light of UX Mon
   with respect to the ephemeris:
   MinI (Hel.) = 2433346.566 + 5.904574d * E

                        [FIGURE 2]
    Figure 2. (O-C)'s of the epochs of minimum light of UX Mon
    (E > -300) with respect to the ephemeris:
    MinI (Hel.) = 2433346.563 + 5.904539d * E

    It seems that at least two changes in the period occurred;
the first one (if real) is abrupt, at about E = -650; moreover
the period before E =~ -700 is longer than after E = -700. It can
be noticed that for E> -300 the epochs are well satisfied by a
linear ephemeris; a weighted least-squares solution gives:

      Min_I(Hel.) = 2433346.563 + 5.904539d E
                         +-.  2  +-.     4


    The corresponding (O-C)'s are shown in Figure 2.



                        FRANCO SCALTRITI

                  Osservatorio Astronomico di Torino
                        10025 Pino Torinese
                         Torino, Italia


References:

Gaposchkin, S. 1953 Harvard College Annals, 113, No. 2. [BIBCODE 1953AnHar.113...67G ]
Lynds, C.R. 1956 Publ. Astron. Soc. Pacific, 68, 339. [BIBCODE 1956PASP...68..339L ]
Scaltriti, F. 1973 Mem.S.A.It., Vol. XLIV, 387. [BIBCODE 1973MmSAI..44..387S ]
Soloviev, A. 1943 Astron. Circ. U.S.S.R., 3, 19.
Struve, O. 1947 Astroph. J., 106, 255. [BIBCODE 1947ApJ...106..255S ]
Whitney, B.S. 1956 Publ.Astron.Soc.Pacific, 68, 253. [BIBCODE 1956PASP...68..253W ]
Whitney, B.S. 1959 Astron.J., 64, 258. [BIBCODE 1959AJ.....64..258W ]