  We report the discovery of four rare debris disks with warm excesses around
F-stars, significantly increasing the number of such systems known in the solar
neighborhood. Three of the disks are consistent with the predictions of steady
state planetesimal disk evolution models. The oldest source, HD 169666,
displays a dust fractional luminosity too high to be in steady state and we
suggest that this system recently underwent a transient event of dust
production. In addition, two spectra of this star separated by ~three years
show silicate emission features, indicative of submicron- to micron-sized
grains. We argue that such small grains would be rapidly depleted and their
presence in both spectra suggests that the production of small dust is
continuous over at least on few years timescale. We predict that systems
showing variable mid-infrared spectra, if exist, will provide valuable help in
distinguishing the possible scenarios proposed for dust replenishment.
