\documentstyle[epsf]{article}
\input ibvs.sty
\def\ubv{\hbox{$U\!BV$}}
\def\ub{\hbox{$U\!-\!B$}}
\def\bv{\hbox{$B\!-\!V$}}

\begin{document}

\IBVShead{4295}{8 February 1996}

\centerline{\ft UBV PHOTOMETRY OF NOVA Cas 1995 AT PREMAXIMUM PHASE}
\vskip 8mm

\begintext

    Nova Cas 1995 was discovered at R.A. = 01\fh 05\fmm 05\fss 4, Dec. =
+54\degr 00\farcmm 41\farcss\ (equinox 2000.0) by Yamamoto (1995) on
August 24, 1995 UT. \ubv\ photoelectric photometry reported here was
made from three days after the discovery to mid-January 1996. The equipments
used were two same
type photon counting photometers attached to  Schmidt-
Cassegrain telescopes whose apertures are 28-cm and 35-cm at different
personal observatories of two of the authors (H.A. and N.O.) 
respectively. The photometers developed by one of the authors are same
type instrument described in Ohshima (1988).

    Actual observations were made differentially with respect to
SAO\,21974 = HD\,6250 as a comparison star, and the comparison was
also checked with SAO\,22031. The integration time was 20-seconds for
each band. No  variation of the comparison
star was detected. Obtained magnitude and colors of the comparison star are
$V$ = 6.81, \bv\ = 0.54 and \ub\ = 0.00 which were differentially measured
with $\mu$ Cas ($V$ = 5.12, \bv\ = 0.69, \ub\ = 0.09, Johnson and
Morgan, 1953). In the data reduction the dead time corrections of the
photon counter and the atmospheric extinction correction were carried
out. The transformation coefficients to the \ubv\ standard system used
were obtained from observation of the standard stars.

  The magnitudes and colors which are mean values obtained during one
night are given in Figure 1a and 1b. The long premaximum halt was
seen during about a hundred days. The magnitude and color curves at
premaximum phase are very similar to that in HR Del (Duerbeck, 1981). The
maximum in U band was earliest. In our data, the maximum is $V$ = 7.08
at December 19, 1995 UT (HJD\,2450071), $B$ = 7.60 and $U$ = 7.69 at
December 17 UT (JD\,2450069).

In color variation, the reddening pulse at the maximum is obviously
observed as same as in other novae (Duerbeck 1981, van den Bergh and
Younger 1987).  The bottom of reddening pulse in \bv\ is later than
that in \ub\, i.e.~ $B-V=+0.80$ on December 21 (JD\,2450073) and $
U-B=+0.52$ on December 19 UT (HJD\,2450071).

  The color-color diagram is given in Figure 2. The color track of
nova light at premaximum halt and at maximum are situated at narrow
linear region on the diagram.  The set of colors at post-maximum light
are situated at separated region.

  In the case of this nova at premaximum halt, the behavior of color
in \ubv photometry was attributed to almost continuum light of the
nova. Because emission lines in the photometric passbands at the phase
of this nova are very weak and narrow. Even though in B band which is
the most affected by emission lines, the contribution from emission to
the band during premaximum halt is less than a few percent, according
to our low dispersion spectra obtained at Bisei Astronomical
observatory (Ohshima et al. 1995) and at Okayama Astrophysical
Observatory (Norimoto, 1996).
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\centerline{2}
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\centerline{\epsffile{4295-f1.ps}}
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\small
\centerline{Figure 1. Magnitude and color curve of Nova Cas 1995}
\large
\vskip 16mm
  According to Gonzalez-Riestra et al. (1996),  the color excess
E(\bv) = 0.6  is obtained from UV observation with IUE. Using this
value, the reddening corrected track on the color-color diagram is
very similar to those of early type supergiants. These colors
are  consistent with observed premaximum spectra of classical novae
(Seitter 1989). 


   The UBV photometric data  reported here are available on request from the 
authors via E-mail (address given at the affiliation).


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\centerline{\epsffile{4295-f2.ps}}
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\small
\noindent Figure 2.
Color-color diagram of Nova Cas 1995.
Filled circles are indicated premaximum halt light, filled triangles
 are post maximum light and large open circle is at  V maximum light.
Dotted line indicates progress of time. Open symbols are after
correction of interstellar reddening which corresponds to E(B$-$V)=0.6.
Colors at premaximum halt are situated narrow region on the
diagram. Reddening corrected colors of light at premaximum halt and
at maximum are very similar to those of early type supergiants.
\large
\vspace{1.5cm}

\beginauthors
\authorsrightcol
OSAMU OHSIMA
ohshima@bao.go.jp                                      %email

Bisei Astronomical Observatory
Bisei, Okayama
714-14, Japan 
\vspace{10pt}
HIDEHIKO AKAZAWA
107 Funao, Okayama
710-02, Japan
\vspace{10pt}
NOBUO OHKURA
25 Senoh, Okayama
701-02, Japan
\endauthors

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\centerline{4}
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\references

Duerbeck, H. W., 1981, {\it Publ. Astron. Soc. Pac.}, {\bf 93}, 165

Gonzalez-Riestra, R., Shore, S. N., Starrfield, S. and Krautter, J.,
1996, {\it IAU Circ.}, No.\,6295

Johnson, H.L. and Morgan, W.W., 1953, {\it Astrophys. J.}, {\bf 117},
313

Norimoto, Y., 1996, private communication

Ohshima, O., 1988, {\it I.B.V.S.}, No.\,3135

Ohshima, O., Ayani, K. Shimizu, M. and  Yamamoto, T., 1995, {\it I.A.U. Circ.}, No.\,6214.

Seitter, W. C., 1989, in {\it Physics of Classical Novae},
Springer-Verlag, ed.~Cassatella, A. and Viotti, R., p.79

van den Bergh, S. and Younger, P.F., 1987, {\it
Astron. Astrophys. Suppl.}, {\bf 70}, 125.

Yamamoto, M., 1995, {\it I.A.U. Circ.}, No.\,6213


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