  Over the last decade, the pre-main sequence star KH 15D has exhibited
periodic eclipses that are surprisingly deep (~3 mag) and long-lasting (~40% of
the 48.4-day period). The cause of the eclipses is unknown, but it could be a
feature in a nearly edge-on protoplanetary disk. Here we report on an analysis
of archival photographs of KH 15D from the Harvard College Observatory plate
collection, most of which were taken during the years 1913-1951. During this
time range, the data are consistent with no eclipses; the duty cycle of 1 mag
eclipses was less than 20%. The decadal timescale of this change in eclipse
behavior is compatible with the expected timescale of protoplanet/disk
interactions. Archival images from more recent epochs should reveal the onset
of the eclipses.
