We propose an ISOCAM/ISOPHOT search at 6.7/15 and 60/100 microns
 for faint circumstellar disks around very young (1-2 Myr) stars
 of low mass in the most nearby OB association (Sco-Cen or Orion
 depending on the launch window). All these stars are weak-line,
 X-ray (EINSTEIN, ROSAT) selected T Tauri stars (WTTS).
 It has been suggested that such stars are truly naked (diskless).
 The sensitivity of ISO is such that we can probe the presence of
 disk dust masses up to a factor of 100 lower than ground-based
 observations (i.e. of order earth masses). By concentrating on the
 youngest WTTS, we will find out whether WTTS are genuinely naked
 or whether WTTS only lack the innermost disk and, correspondingly,
 the signatures of accretion (e.g. large Halpha equivalent widths).
 Therefore the proposed observations go a long way to understand
 the difference between weak-line and classical T Tauri stars.
 Indeed we expect to find faint dusty disks and will investigate
 their properties (dust mass, dust temperature) in a well-defined
 nearly coeval sample (each object is placed in the HR-diagram).
 The 7/15 micron data will probe the presence of warm inner disks
 at radii of the order 1 AU, important for the formation of
 terrestrial planets. The 60/100 micron data bear on the presence
 of cold outer disks with a scalelength of the order of 100 AU,
 important for the formation of very low-mass binary companions.
 Models suggest the disks are likely to be optically thin at each
 of the 4 wavelengths, in which case the dust mass of both the
 inner and outer disks can be determined. This will respresent an
 enormous quantitative advance in our understanding of young disks
 (disk dissipation timescale as a function of stellar mass), in an
 environment where most of the field stars are believed to form.