Many energetic \gamma -ray pulsars discovered by Fermi are promising candidates for optical follow-ups .
We present the results of the first deep optical observations of the two Vela-like Fermi pulsars PSR J1357 - 6429 and PSR J1048 - 5832 performed with the VLT .
However , they have not been detected down to V \sim 27 and V \sim 27.6 , respectively ( 3 \sigma ) .
These upper limits suggest an efficiency in converting spin-down power into optical luminosity < 7 \times 10 ^ { -7 } and < 6 \times 10 ^ { -6 } , respectively , lower than the Crab pulsar and , possibly , more compatible with the spin-down age of these two pulsars .