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\IBVShead{4956}{13 September 2000}

\IBVStitletl{First Determinations of Photoelectric Minima,}{ Real Period and Study of the Period of NP P\lowercase{av}}

\IBVSauth{Cerruti, Miguel Angel}

\IBVSinsto{\small{Instituto de Astronom\'{\i}a y F\'{\i}sica del Espacio, CC 67 -- Suc 28, 1428 Buenos Aires, Argentina,\\ \indent e-mail: miguelan@iafe.uba.ar}}

\IBVSabs{The real period of this binary was determined when the light curve}
\IBVSabs{was completed.}
\IBVSabs{Between the photographic and photoelectric minima the period remains}
\IBVSabs{in first approximation constant.}

\SIMBADobj{NP Pav}
\IBVStyp{ EA }
\IBVSkey{photometry, ephemerides, variation of the period, unique systems}

\begintext

NP Pav was discovered by Hoffmeister (1949) who published a finder
chart (1957).  Shaw and Sievers (1970) found that it is an EA object, with a
period of 1\fday266821 and a deep secondary minimum. They also published a
list of minima and a finder chart.

We present here the first photoelectric determinations of minima of the
eclipsing binary NP Pav = S 5117 = KSP 5263 = BV 1305 = GSC\,9321:1055.
The observations were made during three runs, all from Cerro Tololo
Inter-American Observatory\footnote{NOAO with is operated by AURA
Inc.\ under cooperative agreement with the NSF} in Chile with the Lowell
telescope and single-channel photon counting techniques and standard
$UBV$ filters. In 1982 and 1984 a refrigerated phototube EMI 2070 was
used while in 1995 a refrigerated phototube RCA 31034A was utilized.
GSC\,9308:1513 = CPD\,$-$69\deg3134 (9\fmm6) served as the comparison
and GSC\,9321:1105 = CPD\,$-$69\deg3146 (6\fmm7) = CoD\,$-$69\deg1958
(7\fmm1) = SAO\,254907 = HD 198971 (B9/9.5V 6\fmm9) as the check.

The photoelectric light curve is completed in the three filters $U$, $B$ and
$V$. It is presented in Figure 1 together with their color index curves
$u-b$ and $b-v$.  The light curve shows a shallow secondary minimum
of 0\fmm225 depth in $V$, therefore the period must be reduced to half
of the value. The depth of the primary minimum is 1\fmm075
in $V$. The eclipse is almost complete and the portion of the light curve
that is included into the eclipse is measured by the external tangent
angle that is 0.12 in phase units. The $b-v$ color is somewhat redder
by about 0\fmm025 in $V$ around the primary minimum and bluer for the
same amount around the secondary minimum. Outside the minima the light
curve is not constant showing the proximity effects.

\begin{table}
\caption{Times of photoelectric minima and residuals for linear ephemeris of NP Pav}
\begin{center}
\begin{tabular}{ccrr@{.}lr}
\hline
\lower6pt\hbox{Min.} &  \lower6pt\hbox{Band} & \multicolumn1c{HJD(sigma)} & \multicolumn2c{\lower6pt\hbox{$E$}} &   \multicolumn1c{\lower6pt\hbox{$O-C$}} \\[-3pt]
&  & \multicolumn1c{2400000\,+} \\
\hline
I  & $U$ & 45984.7094(0.0010) &    0&0 & $-$0.0001 \\
I  & $B$ & 45984.7093(0.0012) &    0&0 & $-$0.0002 \\
I  & $V$ & 45984.7098(0.0014) &    0&0 &    0.0003 \\
II & $U$ & 49945.9004(0.0037) & 6252&5 &    0.0035 \\
II & $B$ & 49945.8995(0.0028) & 6252&5 &    0.0026 \\
II & $V$ & 49945.8995(0.0026) & 6252&5 &    0.0026 \\
I  & $U$ & 49946.8478(0.0010) & 6254&0 &    0.0006 \\
I  & $B$ & 49946.8476(0.0012) & 6254&0 &    0.0004 \\
I  & $V$ & 49946.8478(0.0011) & 6254&0 &    0.0006 \\
II & $U$ & 49947.7943(0.0027) & 6255&5 & $-$0.0032 \\
II & $B$ & 49947.7974(0.0011) & 6255&5 & $-$0.0001 \\
II & $V$ & 49947.7963(0.0017) & 6255&5 & $-$0.0012 \\
I  & $U$ & 49948.7495(0.0011) & 6257&0 &    0.0016 \\
I  & $B$ & 49948.7491(0.0014) & 6257&0 &    0.0012 \\
I  & $V$ & 49948.7487(0.0021) & 6257&0 &    0.0008 \\
I  & $U$ & 49951.9113(0.0038) & 6262&0 & $-$0.0042 \\
I  & $B$ & 49951.9108(0.0018) & 6262&0 & $-$0.0047 \\
I  & $V$ & 49951.9108(0.0012) & 6262&0 & $-$0.0047 \\
II & $U$ & 49959.8388(0.0026) & 6274&5 &    0.0041 \\
II & $B$ & 49959.8357(0.0016) & 6274&5 &    0.0010 \\
II & $V$ & 49959.8354(0.0011) & 6274&5 &    0.0007 \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8cm}{4956-f1.eps}{Complete light and color curve of NP Pav.
The vertical scale corresponds to $\delta v$, the shifts are:  $\delta
b=-0.402$, $\delta u=-0.651$, $\delta(b-v)=-0.711$,
$\delta(u-b)=-0.898$}

\begin{table}
\caption{Times of minima and residuals for parabolic and linear ephemeris of NP Pav}
\begin{center}
\begin{tabular}{cccrr@{.}lrr}
\hline
\lower6pt\hbox{Ref.} & \lower6pt\hbox{Min.} & \lower6pt\hbox{Band} & \multicolumn1c{HJD(sigma)} & \multicolumn2c{\lower6pt\hbox{$E$}} & \multicolumn1c{\lower6pt\hbox{$O-C$}} & \multicolumn1c{\lower6pt\hbox{$(O-C)'$}} \\[-3pt]
& & & \multicolumn1c{2400000\,+} \\
\hline
1 & I & pg. & 38234.4080(0.0200) & $-$12236&0 & $-$0.0037 &    0.0082 \\
1 & I & pg. & 38258.4520(0.0200) & $-$12198&0 & $-$0.0289 & $-$0.0176 \\
1 & I & pg. & 38260.3610(0.0200) & $-$12195&0 & $-$0.0201 & $-$0.0088 \\
1 & I & pg. & 38307.2330(0.0200) & $-$12121&0 & $-$0.0198 & $-$0.0095 \\
1 & I & pg. & 38314.2310(0.0200) & $-$12110&0 &    0.0108 &    0.0209 \\
1 & I & pg. & 38555.5490(0.0200) & $-$11729&0 &    0.0029 &    0.0080 \\
1 & I & pg. & 38562.5120(0.0200) & $-$11718&0 & $-$0.0015 &    0.0035 \\
1 & I & pg. & 38614.4370(0.0200) & $-$11636&0 & $-$0.0155 & $-$0.0115 \\
1 & I & pg. & 38614.4650(0.0200) & $-$11636&0 &    0.0125 &    0.0165 \\
1 & I & pg. & 38621.4280(0.0200) & $-$11625&0 &    0.0081 &    0.0120 \\
1 & I & pg. & 38640.3980(0.0200) & $-$11595&0 & $-$0.0239 & $-$0.0204 \\
1 & I & pg. & 38642.3110(0.0200) & $-$11592&0 & $-$0.0111 & $-$0.0077 \\
1 & I & pg. & 38649.3110(0.0200) & $-$11581&0 &    0.0215 &    0.0248 \\
1 & I & pg. & 38675.2400(0.0200) & $-$11540&0 & $-$0.0190 & $-$0.0162 \\
1 & I & pg. & 38694.2500(0.0200) & $-$11510&0 & $-$0.0110 & $-$0.0086 \\
1 & I & pg. & 39029.3330(0.0200) & $-$10981&0 &    0.0027 & $-$0.0010 \\
1 & I & pg. & 39373.2820(0.0200) & $-$10438&0 &    0.0148 &    0.0054 \\
1 & I & pg. & 39378.3280(0.0200) & $-$10430&0 & $-$0.0064 & $-$0.0159 \\
1 & I & pg. & 39385.3340(0.0200) & $-$10419&0 &    0.0322 &    0.0226 \\
1 & I & pg. & 40089.0310(0.0200) &  $-$9308&0 &    0.0203 &    0.0010 \\
1 & I & pg. & 40096.0070(0.0200) &  $-$9297&0 &    0.0289 &    0.0095 \\
1 & I & pg. & 40419.0310(0.0200) &  $-$8787&0 &    0.0182 & $-$0.0048 \\
1 & I & pg. & 40450.0620(0.0200) &  $-$8738&0 &    0.0126 & $-$0.0107 \\
2 & I & $U$ & 45984.7094(0.0010) &        0&0 & $-$0.0006 & $-$0.0001 \\
2 & I & $B$ & 45984.7093(0.0012) &        0&0 & $-$0.0007 & $-$0.0002 \\
2 & I & $V$ & 45984.7098(0.0014) &        0&0 & $-$0.0002 &    0.0003 \\
\hline
\end{tabular}

\normalsize
References: 1 photographic minima; 2 photoelectric minimum of 1984.
\end{center}
\end{table}

\IBVSfig{6.53cm}{4956-f2.eps}{Behavior of the $O-C$ residuals for NP Pav
from formulae (1) and (2).  Hollow circles stand for primary minima,
vertical bars are for errors}

\vskip-22pt
The photographic minima were scaled to the new period and a dispersion
of 0.02 incorporated to all of these minima. The linear solution is
${\rm Min~I = HJD}~2438234.4014 + 0\fday6334113 \times E$ with an
error of 0\fday0042 for the day and 0\fday0000027 for the period. Two
sets of photoelectric minima, one of only one minimum in 1984 and the
other with six minima in 1995, were derived by the polynomial line
method (Guarnieri et al.\ 1975, Ghedini 1982). A least square solution
for the photoelectric times of minima gives:
\begin{equation}
\begin{array}{r@{}r@{}l}
{\rm Min~I = HJD}~2445984.7095 & {} + 0\fday63353658 & {} \times E \\
{} \pm 0.0011 & {} \pm 0.00000020 & ~{\rm m.e.,} \\
\end{array}
\end{equation}
without a term of the second order, in other words, the period in the
photoelectric part has remained in first approximation constant. In Table~1
are shown the photoelectric minima, the dispersion associated with each
minimum, the epoch numbers and the $O-C$ residuals respect to (1). 
It was not possible to find a common solution for both the photographic
and the photoelectric minima. The photographic and the 1984 photoelectric
minima gives the following formula:
\begin{equation}
\begin{array}{r@{}r@{}r@{}l}
{\rm Min~I = HJD}~2445984.7095 & {} + 0\fday6333894 & {} \times E - 1\fday04 \times 10^{-9} & {} \times E^2 \\
{} \pm 0.0019 & {} \pm 0.0000030 & {} \pm 0.26 \times 10^{-9} \\
\end{array}
\end{equation}
which is quadratic. This is shown in Table~2 that is similar to Table~1, 
where the $O-C$ and $(O-C)'$ are the residuals respect to the linear and
parabolic solution.

Although comparing the periods of the photographic solution
(0\fday6334113[27]) with that corresponding to the photoelectric
solution (0\fday63353658[20]) the period varies and the second order
term in (2) is not negligible, the large errors of the $(O-C)'$ values
implies, that the quadratic fit seems to be not reliable (suggested by
a referee). We consider that in a first approximation the period has
remained constant during all the `history' of this system.  The formula
(1) that is all photoelectric is to be used for derive new times of
minima. The $O-C$ diagram is displayed in Figure~2.

The author would like to thank the staff and Director of CTIO for their
hospitality.

\references

Ghedini, S., 1982, {\it Software for Photometric Astronomy}, Ghedini ed., 
Willman-Bell Inc.\ Press, Richmond, p.\ 59

Guarnieri, A., Bonifazi, A., Battistini, P., 1975, {\it Astron.\ Astrophys.\
Supp.}, {\bf 20}, 199

Hoffmeister, C., 1949, {\it Erg.\ Astronom.\ Nachr.}, {\bf 12}, No.\ 1

Hoffmeister, C., 1957, {\it Mitt.\ Ver\"and.\ Sterne}, {\bf 1}, 245

Shaw, S., Sievers, J., 1970, {\it Bamberg Ver\"off.}, {\bf VIII}, No.\ 90

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