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\IBVShead{5390}{10 March 2003}

\IBVStitletl{New Photoelectric Photometry of}{ the Young Star LO Pegasi}

\IBVSauth{DAL, H. A.; TA\c{S}, G.}

\IBVSinsto{Ege University Observatory, 35100 Bornova, Izmir, Turkey}
\IBVSinsto{e-mail: alidal@astronomy.sci.ege.edu.tr, tas@astronomy.sci.ege.edu.tr}

\SIMBADobj{LO Peg}

\begintext


The single, young K8 V star LO Peg (Bowyer et al., 1996) is a member of the 
Local Association (Jeffries and Jewel, 1993; Montes et al., 2001) and it has a 
high projected 
equatorial velocity of 69 kms$^{-1}$ (Jeffries et al., 1994). LO~Peg is an 
active star showing strong H$\alpha$ and Ca II H, K emission lines 
(Jeffries et al., 1994). It has 
generally  been chosen as a target star for Doppler imaging studies due to
its short period. There are a few photometric studies on this active star up to
 now. So, we observed it as a part of our observing program on the young, solar
 type stars. 

The photometric observations were carried out using SSP-5 photometer equipped 
to the 30 cm Schmidt-Cassegrain type telescope 
of Ege University Observatory between July 24 and October 31, 2002. BD\,+22\deg
 4417 (G0, $V$=9\fmm03) and BD\,+22\deg 4377 
(K0, $V$=8\fmm27) are chosen as the comparison and the check stars, 
respectively. The nightly 
mean differential $V$ magnitudes and standard deviation are 0\fmm806 and 
0\fmm014, ensuring the constancy of both comparison and check stars at this 
level. We obtained 
a total of 655 differential magnitudes (in the sense variable minus comparison)
 in Johnson $B$ and $V$ filters during 18 nights and, corrected for 
atmospheric extinction. The extinction coefficients were calculated for each 
band using the observed magnitudes of the comparison 
star. The times were also reduced to the Sun's center. The standard deviation 
of each observed point is approximately 0\fmm018 in $V$ band and 0\fmm028 in 
$B$.


Jeffries et al. (1994) proposed six probable rotational periods and they stated
 that the periods of 0\fday38417 (9\fhr22) and 0\fday42375 (10\fhr17) are the 
more 
likely periods. Robb and Cardinal (1995) eliminated completely the possibility 
of the 0\fday38417 period. We used the following ephemeris to compute the 
phases of observations:



\begin{center}
HJD = 24 48869.93 + 0\fday42375 $\times$ E
\end{center}

\begin{table}
\centerline{{\bf Table 1.} The properties of the light curves.}
\begin{center}
\begin{tabular}{ccccc}
\hline
Data   & JD range & Amplitude & $\theta_{min}$ (I) & $\theta_{min}$ (II)\\
 Groups    &(24 52000 +)    &	(mag)      &         &      \\
\hline
I & 480 $-$ 488 & 0.080 & 0.8 & 0.1\\
II & 495 $-$ 507 & 0.067 & 0.0 & \\
III & 511 $-$ 512 & 0.070 & 0.0 & 0.7 \\
VI & 536 $-$ 542 & 0.069 & 0.7 & \\
V & 549 $-$ 579 & 0.046 & 0.4 & \\
\hline
\small
\end{tabular}
\end{center}
\end{table}


It appears that the shape and the amplitude of the light curve are changing 
during the observing season. Therefore, we divided our data into five different
 groups. The light curves and $B-V$ colour curves obtained in this study are 
shown in Figure~1. The phases of minima are determined by representing the 
light curves with free hand curve, and the properties of the light curves are 
given in Table~1.


The light curves show wave-like distortion that is a good indicator of 
the spot activity. The colour curves of the first three groups do not indicate 
a significant variation, while those of the last two groups show remarkable 
changes. As can be seen from Figure~1 the light curves have three properties 
clearly seen: (1) Generally, they show strong asymmetry which implies the 
effect
of the second starspot ($\theta_{min}$ II), its traces are quite obvious in 
Groups~I and III. (2) The amplitude of the light curves slightly decreases from
 the beginning of the observing season to the end of it. These amplitudes are 
much lower than those given in the literature (the amplitudes of 0\fmm15 and
 0\fmm20 were given by Jeffries et al. (1994) and Robb and Cardinal (1995), 
respectively). (3) It can be clearly seen that there is a shift at the phase of
 the wave-like distortion. When we compare the light curves presented in 
Groups~I and V, this phase shift is quite evident. Robb and Cardinal (1995) 
mentioned about the amplitude variation on a daily timescale, but there is no 
comment about shifting at the wave minimum in the literature. The 
cycle-to-cycle variation of both the amplitude and the phase of the wave 
minimum were also reported for some rapidly rotating, young stars like FR~Cnc 
(Pandey, 2002), AB~Dor (Bos, 1994; Donati and Collier-Cameron, 1997) and Speedy
 Mic (Barnes et al., 2001). We need more photometric observations to 
determine the activity nature of LO~Peg.

\IBVSfig{20cm}{5390-f1.eps}{The light curves of LO Peg in $V$ band. The data show the average observing points with 0.04 phase intervals.}
%%\IBVSfigKey{5390-f1.eps}{LO Peg}{light curve}

The authors would like to thank Dr. K. Olah for her valuable suggestions.


\references

Barnes, J. R., Collier Cameron, A., James, D. J., Steeghs, D., 2001, {\it MNRAS}, {\bf 326}, 1057

Bos, M., 1994, {\it IBVS}, 4111

Bowyer, S., Lampton, M., Lewis, J., Wu, X., Jelinsky, P., Malina, R. F., 1996, {\it ApJS}, {\bf 102}, 129

Donati, J.-F., Collier Cameron, A., 1997, {\it MNRAS}, {\bf 291}, 1

Jeffries, R. D., Jewell, S. J., 1993, {\it MNRAS}, {\bf 264}, 106

Jeffries, R. D., Byrne, P. B., Doyle, J. G., Anders, G. J., James, D. J., Lanzafame, A. C., 1994, {\it MNRAS}, {\bf 270}, 153

Montes, D., Lopez-Santiago, J., Galvez, M. C., Fernandez-Figueroa, M. J., de Castro, E., Cornide, M., 2001, {\it MNRAS}, {\bf 328}, 45

Pandey, J. C., Singh, K. P., Sagar, R., Drake, A. S., 2002, {\it IBVS}, 5351

Robb, R. M., Cardinal, R. D., 1995, {\it IBVS}, 4221

\endreferences


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