We present spatially resolved near-infrared and mid-infrared ( 1–25 µm ) imaging of the WL 20 triple system in the nearby ( d = 125 pc ) \rho Ophiuchi star-forming cloud core .
We find WL 20 to be a new addition to the rare class of “ infrared companion systems ” , with WL 20 : E and WL 20 : W displaying Class II ( T-Tauri star ) spectral energy distributions ( SEDs ) and total luminosities of 0.61 and 0.39 L _ { \sun } , respectively , and WL 20 : S , the infrared companion , with a Class I ( embedded protostellar ) SED and a luminosity of 1.0–1.8 L _ { \sun } .
WL 20 : S is found to be highly variable over timescales of years , to be extended ( 40 AU diameter ) at mid-infrared wavelengths , and to be the source of the centimeter emission in the system .

The photospheric luminosities of 0.53 L _ { \sun } for WL 20 : E and 0.35 L _ { \sun } for WL 20 : W , estimated from our data , combined with existing , spatially resolved near-infrared spectroscopy , allow us to compare and test current pre-main-sequence evolutionary tracks .
The most plausible , non-accreting tracks describing this system are those of d ’ Antona & Mazzitelli ( 11 ) .
These tracks give an age of 2– 2.5 \times 10 ^ { 6 } yr and masses of 0.62–0.68 M _ { \sun } for WL 20 : E and 0.51–0.55 M _ { \sun } for WL 20 : W , respectively .
The age and mass of WL 20 : S can not be well determined from the currently available data .
WL 20 : E and WL 20 : W fall into the region of the H-R diagram in which sources may appear up to twice as old as they actually are using non-accreting tracks , a fact which may reconcile the co-existence of two T-Tauri stars with an embedded protostar in a triple system .
The derived masses and observed projected separations of the components of the WL 20 triple system indicate that it is in an unstable dynamical configuration , and may therefore provide an example of dynamical evolution during the pre-main-sequence phase .