To cite the service as such, we suggest the following BibTeX entry:
@MISC{vo:purx_enrolling, year=2017, title={purx Enrollment Web Service}, author={Demleitner, M.}, url={http://dc.zah.uni-heidelberg.de/purx/q/enroll/info}, howpublished={{VO} resource provided by the {GAVO} Data Center} }