  A model scenario has recently been introduced to explain the presence of very
strong Li lines in the spectra of some low mass K giant stars (de la Reza et
al. 1996). In this scenario all ordinary, Li poor, K giants become Li rich
during a short time ($\sim 10^5 yr$) when compared to the red giant phase of 5
10^7 yr. In this ``Li period'', a large part of the stars are associated with
an expanding thin circumstellar shell supposedly triggered by an abrupt
internal mixing mechanism resulting in a surface new ^7Li enrichment. This
letter presents near 40 Li rich K giants known up to now. The distribution of
these Li rich giants, along with other 41 observed K giants that have shell,
but are not Li rich, in a color-color IRAS diagram confirms this scenario,
indicating, also as a new result, that a rapid Li depletion takes place on a
time scale of between $\sim 10^3$ and 10^5 yr. This model explains the problem
of the presence of K giants with far infrared excesses presented by Zuckerman
et al. (1995). Other present and future tests of this scenario are briefly
discussed.
