Context : Characterising stellar and circumstellar properties of embedded young stellar objects ( YSOs ) is mandatory for understanding the early stages of the stellar evolution .
This task requires the combination of both spectroscopy and photometry , covering the widest possible wavelength range , to disentangle the various protostellar components and activities .

Aims : As part of the POISSON project ( Protostellar Optical-Infrared Spectral Survey On NTT ) , we present a multi-wavelength spectroscopic and photometric investigation of embedded YSOs in L 1641 , aimed to derive the stellar parameters and evolutionary stages and to infer their accretion properties .

Methods : Our multi-wavelength database includes low-resolution optical-IR spectra from the NTT and Spitzer ( 0.6-40 \mu m ) and photometric data covering a spectral range from 0.4 to 1100 \mu m , which allow us to construct the YSOs spectral energy distributions ( SEDs ) and to infer the main stellar parameters ( visual extinction , spectral type , accretion , stellar , bolometric luminosity , mass accretion and ejection rates ) .

Results : The NTT optical-NIR spectra are rich in emission lines , which are mostly associated with YSO accretion , ejection , and chromospheric activities .
A few emission lines , prominent ice ( H _ { 2 } O and CO _ { 2 } ) , and amorphous silicate absorption features have also been detected in the Spitzer spectra .
The SED analysis allows us to group our 27 YSOs into nine Class I , eleven Flat , and seven Class II objects .
However , on the basis of the derived stellar properties , only six Class I YSOs have an age of \sim 10 ^ { 5 } yr , while the others are older ( 5 \times 10 ^ { 5 } –10 ^ { 6 } yr ) , and , among the Flat sources , three out of eleven are more evolved objects ( 5 \times 10 ^ { 6 } –10 ^ { 7 } yr ) , indicating that geometrical effects can significantly modify the SED shapes .
Inferred mass accretion rates ( \dot { M } _ { acc } ) show a wide range of values ( 3.6 \times 10 ^ { -9 } to 1.2 \times 10 ^ { -5 } M _ { \sun } yr ^ { -1 } ) , which reflects the age spread observed in our sample well .
Average values of mass accretion rates , extinction , and spectral indices decrease with the YSO class .
The youngest YSOs have the highest \dot { M } _ { acc } , whereas the oldest YSOs do not show any detectable jet activity in either images and spectra .
Apart from the outbursting source # 25 and , marginally , # 20 , none of the remaining YSOs is accretion-dominated ( L _ { acc } > L _ { * } ) .
We also observe a clear correlation among the YSO \dot { M } _ { acc } , M _ { * } , and age .
For YSOs with t > 10 ^ { 5 } yr and 0.4 M _ { \sun } \leq M _ { * } \leq 1.2 M _ { \sun } , a relationship between \dot { M } _ { acc } and t ( \dot { M } _ { acc } \propto t ^ { -1.2 } ) has been inferred , consistent with mass accretion evolution in viscous disc models and indicating that the mass accretion decay is slower than previously assumed .
Finally , our results suggest that episodic outbursts are required for Class I YSOs to reach typical classical T Tauri stars stellar masses .

Conclusions :