Context :

Aims : We study the non-thermal jet emission of the BL Lac object B3 2247+381 during a high optical state .

Methods : The MAGIC telescopes observed the source during 13 nights between September 30th and October 30th 2010 , collecting a total of 14.2 hours of good quality very high energy ( VHE ) \gamma -ray data .
Simultaneous multiwavelength data was obtained with X-ray observations by the Swift satellite and optical R-band observations at the KVA-telescope .
We also use high energy \gamma -ray ( HE , 0.1 GeV–100 GeV ) data from the Fermi satellite .

Results : The BL Lac object B3 2247+381 ( z=0.119 ) was detected , for the first time , at VHE \gamma -rays at a statistical significance of 5.6 \sigma .
A soft VHE spectrum with a photon index of -3.2 \pm 0.6 was determined .
No significant short term flux variations were found .
We model the spectral energy distribution using a one-zone SSC-model , which can successfully describe our data .

Conclusions :