While periods were detected on 59 of 105 stars monitored only 12, ten in Orion OB1a and OB1b plus TAP 26 and P1724, had detections with FAP's below 1%. This seems consistent with results from the Van Vleck Observatory. At the Van Vleck observatory, they obtain data about 35 nights a year over the entire winter season, but they have a somewhat smaller aperture than the 1 meter used for the bulk of the results presented here. The Van Vleck Observatory has reported periods for 75 stars (Choi & Herbst 1996) and variability has been seen in 186 stars (Attridge & Herbst 1992) over four years. In general, they report that they can only measure rotation periods for about 10% of their target stars in any given season (Herbst 1996). It is unclear what fraction 186 is of the total number of Jones & Walker (1988) objects in their fields.
Several factors contributed in keeping the ratio of monitored stars to detected periods low. First of all, as demonstrated in the previous section, the variability of the stars can be complicated, especially when inclination effects and flares are included. Some stars had very little signal; at least 20% of the X--ray sources monitored exhibited variations which were less than the mean noise level in the field standard stars.
Exposures were calculated so that the brightest X--ray sources, usually about 11th magnitude at V, would be well exposed. Unfortunately, this causes fairly large errors in dimmer stars. The brightest X--ray sources themselves usually showed very little variability since the bulk of these stars are G7 and earlier. At the other end of the scale, some of the sources showed huge variability of well over a magnitude. These could be early--type T Tauri stars (ETTS; Herbst et al.\ 1994) or cTTs with highly variable accretion.