Aleksi? J., Anderhub H., Antonelli L.A, Antoranz P., Backes M., Baixeras C., Balestra S., Barrio J.A, Bastieri D., J. González B et al..  2010.  Simultaneous multi-frequency observation of the unknown redshift blazar PG 1553+113 in March-April 2008. Astronomy and Astrophysics. 515:A76.