  The Archives of Photographic PLates for Astronomical USE (APPLAUSE) project
is aimed at digitising astronomical photographic plates from three major German
plate collections, making them accessible through integration into the
International Virtual Observatory (IVO). Photographic plates and related
materials (logbooks, envelopes, etc.) were scanned with commercial flatbed
scanners. Astrometric and photometric calibrations were carried out with the
developed PyPlate software, using Gaia EDR3 data as a reference. The APPLAUSE
data publication complies with IVO standards. The latest data release contains
images and metadata from 27 plate collections from the partner institutes in
Hamburg, Bamberg, and Potsdam, along with digitised archives provided by
Tautenburg, Tartu, and Vatican observatories. Altogether, over two billion
calibrated measurements extracted from about 70,000 direct photographic plates
can readily be used to create long-term light curves. For instance, we
constructed the historic light curve of the enigmatic dipping star KIC 8462852.
We found no evidence of previously assumed variations on timescales of decades
in our light curve. Potential uses of APPLAUSE images for transient sources can
be appreciated by following the development of the nova shell of GK Per (1901)
over time and the change in brightness of two extragalactic supernovae. The
database holds about 10,000 spectral plates. We made use of objective prism
plates to follow the temporal changes of Nova DN Gem through 1912 and 1913,
highlighting an outburst in early 1913.
