Pointed EINSTEIN observations of TTs in the Taurus--Auriga star forming region (SFR) detected five serendipitous sources in the field (Feigelson & Kriss 1981 and Walter & Kuhi 1981). Follow--up ground based observations showed these to be K stars with weak H lines and strong Li I 6707Å absorption lines. Lithium absorption lines are indicators of youth because Li is a fairly fragile element and is destroyed at the temperatures experienced at the base of the convection zone. Lithium is depleted in stars as they age (Duncan 1981). Mundt et al. (1983) showed that these serendipitous sources were radial velocity members of the Taurus--Auriga SFR, but did not exhibit the IR excesses previously associated with TTs.
Further observations in Taurus, Ophiuchus, and Corona Australis showed that these five serendipitous sources were not unique. There exists a class of PMS stars, located in SFRs, which have similar space motions, masses and ages to those of TTs. They lack the strong line emission and infrared excesses which made such objects easy to distinguish observationally. Walter (1986) called these objects naked T Tauri stars (nTTs; Walter 1986, Walter et al. 1987). The nTTs are still on their convective tracks and thought to be more evolved than (although coeval with) the other TTs, which are now referred to as classical T Tauri stars (cTTs). Classical T Tauri stars are viewed as composite systems of a stellar photosphere, an accretion zone and a circumstellar disk of cool material (Bertout et al. 1989).
The catalog of Herbig & Bell (1988) distinguished between cTTs and weak--lined T Tauri stars (wTTs) instead of nTTs. Whereas Walter (1986) defined nTTs on a multi-wavelength basis (lack of accretion and disk signatures in the optical and IR), Herbig & Bell defined wTTs as PMS stars with H equivalent widths of < 10 Å. While all nTTs are wTTs, the converse is not true. There are two type of wTTs which are not nTTs. Higher mass G stars have high continuum fluxes which can hide the emission from the H line. Also, stars with optically thick passive disks do not necessarily show strong H emission, but are not considered nTTs.