Context :

Aims : The properties of the very high energy ( VHE ; E > 100 GeV ) \gamma -ray emission from the high-frequency peaked BL Lac PG 1553+113 are investigated .
An attempt is made to measure the currently unknown redshift of this object .

Methods : VHE Observations of PG 1553+113 were made with the High Energy Stereoscopic System ( HESS ) in 2005 and 2006 .
H+K ( 1.45 - 2.45 \mu m ) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI , an integral field spectrometer of the ESO Very Large Telescope ( VLT ) in Chile .

Results : A VHE signal , \sim 10 standard deviations , is detected by HESS during the 2 years of observations ( 24.8 hours live time ) .
The integral flux above 300 GeV is ( 4.6 \pm 0.6 _ { stat } \pm 0.9 _ { syst } ) \times 10 ^ { -12 } cm ^ { -2 } s ^ { -1 } , corresponding to \sim 3.4 % of the flux from the Crab Nebula above the same threshold .
The time-averaged energy spectrum is measured from 225 GeV to \sim 1.3 TeV , and is characterized by a very soft power law ( photon index of \Gamma = 4.5 \pm 0.3 _ { stat } \pm 0.1 _ { syst } ) .
No evidence for any flux or spectral variations is found on any sampled time scale within the VHE data .
The redshift of PG 1553+113 could not be determined .
Indeed , even though the measured SINFONI spectrum is the most sensitive ever reported for this object at near infrared wavelengths , and the sensitivity is comparable to the best spectroscopy at other wavelengths , no absorption or emission lines were found in the H+K spectrum presented here .

Conclusions :