We investigated the MeV properties of 173 unidentified or only tentatively identified EGRET sources listed in the third EGRET catalogue , by analyzing the simultaneously collected COMPTEL MeV data for each individual source .
The sources can generally be divided into 4 groups .
In this paper we focus on one of these , a group of 22 EGRET sources for which we can provide additional constraining information : their spectral extrapolations from the energy range above 100 MeV towards lower energies overshoot the fluxes or upper limits derived simultaneously at MeV energies .
This means that for these sources a spectral turnover/break between 1 MeV and 100 MeV is required .
At least two of these sources , but most likely the majority of this sample , have the maxima of their gamma-ray luminosities in this energy band .
The sources have rather soft EGRET spectra ( average photon index = 2.72 ^ { +0.08 } _ { -0.11 } ) , and seem to spatially cluster in the inner Galaxy .
Variability analyses revealed 11 out of the 22 sources to be significantly variable .
Object classes proposed as possible counterparts for the unidentified EGRET sources are discussed in the light of these additional constraints .