COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1818

                                  Konkoly Observatory
                                  Budapest
                                  1980 July 18




  UBV PHOTOMETRY OF THE NOVA CYGNI 1975 ON SEPTEMBER 1975
            A PHOTOELECTRIC LIGHTCURVE




    Photoelectric observations of the Nova Cygni 1975 in the
UBV-international system have been carried out at the Stephanion
Observatory using the 30-inch Cassegrain reflector equipped with
a dual channel photoelectric photometer. The photomultiplier
used was an RCA 1P21 refrigerated with dry ice. The transformation 
of our instrumental ubv system to the international UBV
system is given by the following equations:
          V = v_o + 0.059(b-v)_o+ 2.368,
        B-V = 0.737 + 1.035(b-v)_o,
        U-B =-1.675 + 1.122(u-b)_o.
One or two differential measurements of the nova were done each
clear night from September 22 up to September 30, 1975. The
nearby star BD +47d3322 was selected as a comparison star, because 
of its resemblance in magnitude and colour as well with the
nova at that stage. For the best accuracy of our measurements,
provision was made for the evaluation of the second order extinction 
coefficient at the moment of each nova observation and
for the best estimation of the magnitude and colour indices
of the comparison star in use. To this purpose a second red
comparison star was involved in the scheme of the observation.
This star was BD +47d3321, an M star, visual companion of the
star BD +47d3322. Finally, in order to deduce the magnitudes
and the colour indices of these two comparison stars, two well
known standard stars epsilon Cyg and alpha Del (Johnson et al. 1966) were
measured more than twice each observational night.
    The magnitudes and colour indices of the comparison stars,
computed as the weighted means of the respective values which



have been derived from all the observations of these stars during 
the whole period, are given in Table I,

                   Table I
   Star          V   s_V    B-V. s_{B-V}  U-B      s_{U-B}
 BD +47d3322  6.456 0.009 +0.010  0.007  -0.381    0.007
 BD +47d3321  7.306 0.006 +1.549  0.004  +1.095    0.009

From the small r.m.s. errors we may deduce that both stars show
no variability, at least for that period when our observations
of the Nova Cygni 1975 were made.
The magnitudes and colour indices of the Nova Cygni 1975
for the respective time of observation are given in Table II.

            Table II
1975 U.T.   J.D.       V         B-V      U-B
September   2442600+
22.87       78.365     7.455   +0.096   -0.519
22.91       78.409     7.413   +0.105   -0.482
24.86       80.364     7.581   +0.080   -0.442
24.93       80.425     7.581   +0.100   -0.457
25.92       81.416     7.625   +0.064   -0.449
27.86       83.362     7.785   +0.040   -0.446
28.81       84.310     7.845   +0.036   -0.439
28.95       84.449     7.854   +0.023   -0.382
30.95       86.444     7.972   +0.005   -0.439

The r.m.s. errors for the V magnitude and the B-V, U-B colour
indices are s_V=+-0.009, s_{B-V}=+-0.007 and s_{U-B}=+-0.007.
It is well known that small differences in the visual
passband between two instrumental systems result into significant
differences in the deduced V magnitude of a nova, due to the
strong and broad emission lines (H_alpha, H_beta and N_1, N_2 of [OIII],
mainly). Therefore, in order to construct an homogeneous lightcurve, 
we have used the published V magnitudes by Margrave and
Doolittle (1975), French (1975), Piirola (1975) and Williamon
(1977) together with the V magnitudes of this work. All these
observations show no systematic differences in overlapping time
intervals, where the emission lines dominate in the spectrum of
the Nova Cygni 1975. For the early phases of the nova outburst
i.e. before the 4.00 U.T. of September no such selection need to
be done, so we have used the published  V-magnitudes of  the
Nova Cygni 1975 in the IAU Circulars Nos.2826,2828,2832,2839.
By a least square fitting we find that the V lightcurve


of the Nova Cygni 1975 may be represented by the following
equations

  t<= - 1d   V = -6.104t - 4.463 (7)
  -1d<t<3d   V = 1.83 exp(0,29|t|) ) (2)
  3d<t<125d  V = 2.954 + 3.392 log t (3)
  125d<t<280d V = 0.603 + 4.499 log t (4)
In these equations t is expressed in days and is measured from
the time of the maximum brightness i.e. 31.00 U.T. of August,
1975 (JD 2442655.5). Equation (4) holds 1000 days after maximum.
At that time the observed V magnitude of the nova is 14.14mag.
(Kleine et al. 1979) while the calculated V magnitude from this
equation is 14.10 magnitude. If we assume that the equation
(4) is valid until the end of the nova eruption, then the star
will reach its prenova magnitude i.e. 27m about 94 years after
the time of maximum brightness,


                   [FIGURE 1]

The Figure displays the lightcurve of Nova Cygni 1975. The points
represent the observed V magnitudes and the continuous lines the
four equations.




  M. E. CONTADAKIS
Department of Geodetic Astronomy
  University of Thessaloniki

References:
Burkhead, M.S. : 1975, IAU Circ. No.2832 [BIBCODE 1975IAUC.2832....1F ]
French, H., Margrave, T.E., Doolittle, J.H.: 1975,IAU Circ.No.2839 [BIBCODE 1975IAUC.2839....1H ]
Johnson, H.L., Mitchell, R.I., Iriarte, B., Wisniewski, W.Z., :
      1966, Lunar and Planetary Lab. Commun. 4, No. 63 [BIBCODE 1966CoLPL...4...99J ]
Kozai, Y., Florkowski, D.R., Uckotter, D., Fernie, J.D., Piirola,
      V.: 1975, IAU Circ. No. 2834 [BIBCODE 1975IAUC.2834....1K ]
Kleine, T., Kohoutek, L.: 1979, Astron.Astrophys., Vol. 76, 133 [BIBCODE 1979A&A....76..133K ]
Martynov, D.Ya., Tempesti, P.: 1975, IAU Circ. No. 2826 [BIBCODE 1975IAUC.2826....1H ]
Piirola, V.: 1975, IAU Circ. No. 2828 [BIBCODE 1975IAUC.2828....1P ]
Williamon, R.M.: 1977, Publ.Astron.Soc.Pacific, Vol.89, 44 [BIBCODE 1977PASP...89...44W ]