We present results from the most recent ( 2002–2004 ) observing campaigns of the eclipsing system KH 15D , in addition to re-reduced data obtained at Van Vleck Observatory ( VVO ) between 1995 and 2000 .
Phasing nine years of photometric data shows substantial evolution in the width and depth of the eclipses .
The most recent data indicate that the eclipses are now approximately 24 days in length , or half the orbital period .
These results are interpreted and discussed in the context of the recent models for this system put forward by Winn et al .
( 2004 ) and Chiang & Murray-Clay ( 2004 ) .
A periodogram of the entire data set yields a highly significant peak at 48.37 \pm 0.01 days , which is in accord with the spectroscopic period of 48.38 \pm 0.01 days determined by Johnson et al .
( 2004 ) .
Another significant peak , at 9.6 days , was found in the periodogram of the out-of-eclipse data at two different epochs .
We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion .
If so , application of Hut ’ s ( 1981 ) theory implies that the eccentricity of the orbit is e = 0.65 \pm 0.01 .
Analysis of the UVES/VLT spectra obtained by Hamilton et al .
( 2003 ) shows that the v \thinspace sin ( i ) of the visible star in this system is 6.9 \pm 0.3 km s ^ { -1 } .
Using this value of v \thinspace sin ( i ) and the measured rotation period of the star , we calculate the lower limit on the radius to be R = ( 1.3 \pm 0.1 ) R _ { \odot } , which concurs with the value obtained by Hamilton et al .
( 2001 ) from its luminosity and effective temperature .
Here we assume that i = 90 \arcdeg since it is likely that the spin and orbital angular momenta vectors are nearly aligned .
One unusually bright data point obtained in the 1995/1996 observing season at VVO is interpreted as the point in time when the currently hidden star ( B ) made its last appearance .
Based on this datum , we show that star B is 0.46 \pm 0.03 mag brighter than the currently visible star A , which is entirely consistent with the historical light curve ( Johnson et al .
2005 ) .
Finally , well-sampled V _ { J } and I _ { J } data obtained at the CTIO/Yale 1-m telescope during 2001/2002 show an entirely new feature : the system becomes bluer by a small but significant amount in very steady fashion as it enters eclipse and shows an analogous reddening as it emerges from eclipse .
This suggests an extended zone of hot gas located close to , but above , the photosphere of the currently visible star .
The persistance of the bluing of the light curve shows that its length scale is comparable to a stellar radius .