COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 3854

                                             Konkoly Observatory
                                             Budapest
                                             2 March 1993
                                             HU ISSN 0374 - 0676


             THE PERIOD CHANGE OF RT Aur: AN UPDATE


RT Aur (HR 2332) is a 3.73d classical Cepheid which is of some interest
because it occupies a point in the Cepheid instability strip on the HD diagram
that is very nearly the same as that occupied by Polaris.  The two stars are
both naked-aye objects of very low reddening, have the same value of <B-V>_0,
(Fernie 1990) and nearly the same period (3.97d for Polaris.) A remarkable
difference between them, however, is that RT Aur has a V-mag amplitude of 0.8
mag, while Polaris now has an amplitude <= 0.01 mag (Fernie, Kamper, and
Seager, in press.) As detailed in Fernie et al, they are not at the edge of the
instability strip.

In pursuit of other similarities or dissimilarities between the stars, I took
note of Polaris having a well-documented rate of period increase (Arrelano
Ferro 1983) and decided to examine the period change, if any, of RT Aur.  This
had already been done by Szabados (1977), who compiled O-C data going back to
1897 which suggested the star had shown a small period jump around JD 2430000,
but that otherwise time period(s) had been constant.  Szabados' own data,
however, are now 20 years old, and since then new photoelectric data or sources
not then available to Szabados have been obtained.  I detail these below and
combine them with the earlier data to update our knowledge of RT Aur's period
change.

The additional data I have used are those of Kelsall (1971), Feltz and McNamara
(1980), Evans (1976), Moffett and Barnes (1985), and Eggen (1985).  I used the
well-defined lightcurve of Moffett and Barnes as a template to fit to the other
data sets where these were less complete.  The observed times of maximum V
light so obtained were:

          Kelsall:   2438692.624 +-0.041   E = -813
Feltz &  McNamara:   2440131.746 +-0.048       -427
            Evans:   2440970.569 +-0.041       -202
 Moffett & Barnes:   2444079.920 +-0.015       +632
            Eggen:   2444523.920 +-0.015       +751

The cycle count, E, is on Szabados' system.

Combining these results with the other 66 in Szabados' Table 23, and assigning
a weight of 3 to photoelectric determinations and 1 to visual determinations,
the best fitting linear ephemeris was JDmax= 2441723.726 + 3.7282205 E.  The
O-C residuals from this fit, although small and with considerable scatter, show
a trend suggestive of a higher-order term being present.  I have formed normal
points in bins of 2000 cycles, and these are shown in Figure 1 with a
second-order polynomial fitted by least-squares.  This leads to an ephemeris
for the time of maximum V light that is

       JD = 2441723.663 + 3.728166 E - 8.3*10^-9 E^2
                  +- 19         12     1.8


                             [FIGURE 1]



RT Aur thus has a rate of period change, dP/P = -4.6 · 10^-7 or -0.14 sec/yr.
This is among the smaller values known, although not as small as the value of
-0.089 sec/yr known for delta Cep.  By contrast Polaris shows a value of +3.3
sec/yr:  The two stars do, therefore, differ significantly in their rate of
period change.  As discussed in Fernie et al, however, this is unlikely to be
related to their relative amplitudes.

This work was supported in part through an operating grant from the Natural
Sciences and Engineering Research Council of Canada.



                                   J.D. Fernie
                                   David Dunlap Observatory
                                   University of Toronto
                                   Box 360
                                   Richmond Hill, Ont. L4C 4Y6
                                   Canada.

REFERENCES

Arrelano Ferro, A. 1983, ApJ, 274, 755 [BIBCODE 1983ApJ...274..755A ]
Eggen, O.J. 1985, AJ, 90, 1297 [BIBCODE 1985AJ.....90.1297E ]
Evans, N.R. 1976, ApJS, 32, 399 (and 35, 395)
			[BIBCODE 1976ApJS...32..399E ]
			[BIBCODE 1977ApJS...35..395R ]
Feltz, K.A., and McNamara, D.H. 1980, PASP, 92, 609 [BIBCODE 1980PASP...92..609F ]
Fernie, J.D. 1990, ApJ, 354, 295. [BIBCODE 1990ApJ...354..295F ]
Kelsall, T. 1971, GSFC Preprint X-641-71-184 (PhD thesis Univ. of Maryland) [BIBCODE 1971PhDT.........1K ]
Moffett, T.J., and Barnes III, T.G. 1985, ApJS, 58, 843 [BIBCODE 1985ApJS...58..843M ]
Szabados, L. 1977, Mitt. Sternwarte Ungarischen Ak. Wissenschaften, No. 70 
					[BIBCODE 1977CoKon..70....1S ]