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\IBVShead{4664}{26 January 1999}

\IBVStitle{Light Curves for Nova S\lowercase{gr} 1998 and Nova S\lowercase{co} 1998}

\IBVSauth{William Liller$^1$, Albert F. Jones$^2$}

\IBVSinst{Instituto Isaac Newton, Casilla 5022, Re\~naca, Chile, e-mail: wliller@compuserve.com}

\IBVSinst{Carter Observatory, 31 Ranui Road, Stoke, Nelson, New Zealand, e-mail: afjones@voyager.co.nz}

\SIMBADobj{Nova Sgr 1998}
\SIMBADobj{Nova Sco 1998}

\begintext

In 1998 two bright novae were discovered by Liller, one in Sagittarius
on Mar.\ 22.3 UT  and one in Scorpius on Oct.\ 21.0 UT (Liller 1998a
and b, respectively). Immediately following the discoveries, the
authors began photometric observations, Jones visually and Liller with
a CCD and a ``minus-IR'' filter.  As noted elsewhere (Liller \& Jones
1996), this combination of CCD and filter results in a broad band V
system which extends from a wavelength of about 450~nm to 730~nm and
thus includes the H$\alpha$ line.  As noted in the Circulars, the
discovery photographs were made using Kodak Technical Pan film plus an
orange filter which yields a passband extending from approximately 610
nm to 690~nm.

The light curves, shown in Figures 1 and 2, are similar in that both show a steady decline.  However, they clearly differ in two other
respects:  N~Sco was much ``faster'' than N~Sgr; and while the visual and
CCD magnitudes for Nova Sgr never depart by more than a few tenths of a
magnitude, those for Nova Sco began to diverge conspicuously
immediately after discovery.

Clearly, the quantity $t_3$, the time in days that it takes a nova to
decline by three magnitudes from peak brightness, depends on the
passband of the observation.  This annoying fact was emphasized in the
classic works of Arp (1956) and of Payne-Gaposchkin (1957).  The
relationship between $t_3$ and absolute magnitude has been calibrated among others by Arp (1956) and Rosino (1964), who used
blue-sensitive photographs in their work.  Consequently, H$\alpha$,
nearly always the strongest emission feature in nova spectra
especially shortly after peak brightness, did not contribute to their
measurements.

In the blue photographic band, spectra of novae are usually dominated by
numerous fainter emission lines including the higher members of the
Balmer series, and by the continuum, bluish in the absence of
interstellar extinction.  As for the dark-adapted eye most sensitive
near 500~nm, it has little sensitivity at the wavelength of H$\alpha$.
As a result, values of $t_3$ derived from visual observations should
usually agree quite well with the ``classical'' values.


On the other hand, both CCDs and Kodak's Technical Pan emulsion have
their sensitivities very close to 656~nm, the wavelength of H$\alpha$.
One would suspect, therefore, that the intensity of H$\alpha$ relative to
the neighboring continuum would be the root of the cause for the
differing behaviors of the two novae of 1998.  Indeed, low-resolution
spectrograms taken with a CCD and an objective prism by Liller (1998a)
showed for Nova Sgr an H$\alpha$ line in  emission at a level $\sim1.5$ times
brighter than the surrounding continuum, while for Nova Sco, he
reported a broad, intense H$\alpha$ emission at a level of $\sim7.6$ times
brighter than the surrounding continuum (Liller 1998b).

\IBVSfig{8.5cm}{4664-f1.ps}{Light curves of Nova Sgr 1998 showing the
visual magnitude estimates as triangles and the CCD broadband V
observations as circles.  The photographic discovery is indicated with
a square; the "v" denotes a fainter-than pre-discovery observation.}

\IBVSfig{8.5cm}{4664-f2.ps}{Light curves of Nova Sco 1998 using the
same symbols as in Fig 1.  Additionally, plus signs (+) indicate
several estimates kindly provided by the AAVSO.}

\vskip -22pt
To our knowledge there have been no reports of magnitude measurements
in the R and B bands.

From our light curves we find that for Nova Sgr, $t_3 \approx 35$ and
48 from the visual and the CCD observations, respectively.  For Nova
Sco, we estimate $t_3 \approx 12$ and 22 days, respectively.  As usual,
the peak magnitudes are uncertain, but it seems probable that for Nova
Sgr, discovery was made before maximum brightness.  However, for Nova
Sco there remains considerable uncertainty: the previously known
photographs of the region were taken 4 nights before the discovery
photograph, and the extrapolations of the light curves provide only a
hint of what the peak magnitude might have been.

We are most indebted to Drs.\ Nikolai Samus and Hilmar Duerbeck for
urging us to publish light curves of novae, and to Dr.\ Samus for
reading an earlier version of this paper.  It is our intention to
publish more nova light curves in the near future.

\references

Arp, H.C.: 1956, {\it Astr.\ J.}\ {\bf 61}, 15

Liller, W.: 1998a, IAU Circ.\ 6846

Liller, W.: 1998b, IAU Circ.\ 7034

Liller, W., Jones, A.F.: 1996, IBVS No.\,4403

Payne-Gaposchkin, C.: 1957, The Galactic Novae, North-Holland Publ.\ Co.,
Amsterdam, pp.\ 20--24

Rosino, L.: 1964, {\it Ann.\ Ap.}\ {\bf 27}, 497

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