Physical studies of asteroids depend on an availability of lightcurve data .
Targets that are easy to observe and analyse naturally have more data available , so their synodic periods are confirmed from multiple sources .
Also , thanks to availability of data from a number of apparitions , their spin and shape models can often be obtained , with a precise value of sidereal period and spin axis coordinates .

Almost half of bright ( H \leq 11 mag ) main-belt asteroid population with known lightcurve parameters have rotation periods considered long ( P \geq 12 hours ) and are rarely chosen for photometric observations .
There is a similar selection effect against asteroids with low lightcurve amplitudes ( a _ { max } \leq 0.25 mag ) .
As a result such targets , though numerous in this brightness range , are underrepresented in the sample of spin and shape modelled asteroids .
In the range of fainter targets such effects are stronger .
These selection effects can influence what is now known about asteroid spin vs. size distribution , on asteroid internal structure and densities and on spatial orientation of asteroid spin axes .

To reduce both biases at the same time , we started a photometric survey of a substantial sample of those bright main-belt asteroids that displayed both features : periods longer than 12 hours , and amplitudes that did not exceed 0.25 magnitude .
First we aim at finding synodic periods of rotation , and after a few observed apparitions , obtaining spin and shape models of the studied targets .

As an initial result of our survey we found that a quarter of the studied sample ( 8 out of 34 targets ) have rotation periods different from those widely accepted .
We publish here these newly found period values with the lightcurves .

The size/frequency plot might in reality look different in the long-period range .
Further studies of asteroid spins , shapes , and structure should take into account serious biases that are present in the parameters available today .
Photometric studies should concentrate on such difficult targets to remove the biases and to complete the sample .