We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum ( F.S . )
objects in the Perseus , Taurus , Chamaeleon I and II , \rho Ophiuchi , and Serpens dark clouds .
These objects represent a subset of the young stellar objects ( YSOs ) from our previous near-infrared multiplicity surveys .
We detected 45/48 ( 94 % ) of the single sources , 16/16 ( 100 % ) of the primary components , and 12/16 ( 75 % ) of the secondary/triple components of the binary/multiple objects surveyed .
One target , IRS 34 , a 0 \farcs 31 separation F.S .
binary , remains unresolved at near-infrared wavelengths .
The composite spectral energy distributions ( SEDs ) for all of our sample YSOs are either Class I or F.S. , and , in 15/16 multiple systems , at least one of the individual components displays a Class I or F.S .
spectral index .
However , the occurrence of mixed pairings , such as F.S .
with Class I , F.S .
with Class II , and , in one case , an F.S .
with a Class III ( Cha I T33B ) , is suprisingly frequent .
Such behavior is not consistent with that of multiple systems among T Tauri stars ( TTS ) , where the companion of a classical TTS also tends to be a classical TTS , although other mixed pairings have been previously observed among Class II YSOs .
Based on an analysis of the spectral indices of the individual binary components , there appears to be a higher proportion of mixed Class I/Flat-Spectrum systems ( 65-80 % ) than that of mixed Classical/Weak Lined T Tauri systems ( 25-40 % ) , demonstrating that the envelopes of Class I/Flat-Spectrum systems are rapidly evolving during this evolutionary phase .
In general , the individual binary/multiple components suffer very similar extinctions , A _ { v } , suggesting that most of the line-of-sight material is either in the foreground of the molecular cloud or circumbinary .
We report the discovery of a steep spectral index secondary companion to ISO-Cha I 97 , detected for the first time via our mid-infrared observations .
In our previous near-infrared imaging survey of binary/multiple Class I and F.S.YSOs , ISO-Cha I 97 appeared to be single .
With a spectral index of \alpha \geq + 3.9 , the secondary component of this system is a member of a rare class of very steep spectral index YSOs , those with \alpha > + 3 .
Only three such objects have previously been reported , all of which are either Class 0 or Class I .