Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process .
Here we present a study of H \alpha variability in 15 T Tauri and Herbig Ae stars ( K7 - B2 ) over a wide range of time windows , from minutes , to hours , to days , and years .
We assess the variability using linewidth measurements and the time series of line profiles .
All objects show gradual , slow profile changes on time-scales of days .
In addition , in three cases there is evidence for rapid variations in H \alpha with typical time-scales of 10 min , which occurs in 10 % of the total covered observing time .
The mean accretion-rate changes , inferred from the line fluxes , are 0.01 - 0.07 dex for time-scales of < 1 hour , 0.04 - 0.4 dex for time-scales of days , and 0.13 - 0.52 dex for time-scales of years .

In ( ) we derived an upper limit finding that the intermediate ( days ) variability dominated over longer ( years ) variability .
Here our new results , based on much higher cadence observations , also provide a lower limit to accretion-rate variability on similar time-scales ( days ) , thereby constraining the accretion rate variability physics in a much more definitive way .
A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission , although other interpretations are possible as well .
In conjunction with our previous work , we find that the time-scales and the extent of the variability is similar for objects ranging in mass from \sim 0.1 to \sim 5 M _ { \odot } .
This confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars – across a wide range of stellar masses .