Context :

Aims : We investigated 1234 fundamental mode RR Lyrae stars observed by the All Sky Automated Survey ( ASAS ) to identify the Blazhko ( BL ) effect .
A sample of 1547 BL stars from the literature was collected to compare the modulation-period distribution with stars newly identified in our sample .

Methods : A classical frequency spectra analysis was performed using Period04 software .
Data points from each star from the ASAS database were analysed individually to avoid confusion with artificial peaks and aliases .
Statistical methods were used in the investigation of the modulation-period distribution .

Results : Altogether we identified 87 BL stars ( 48 new detections ) , 7 candidate stars , and 22 stars showing long-term period variations .
The distribution of modulation periods of newly identified BL stars corresponds well to the distribution of modulation periods of stars located in the Galactic field , Galactic bulge , Large Magellanic Cloud , and globular cluster M5 collected from the literature .
As a very important by-product of this comparison , we found that pulsation periods of BL stars follow Gaussian distribution with the mean period of 0.54 \pm 0.07 d , while the modulation periods show log-normal distribution with centre at \log ( P _ { m } ~ { } { [ d ] } ) = 1.78 \pm 0.30 dex .
This means that 99.7 % of all known modulated stars have BL periods between 7.6 and 478 days .
We discuss the identification of long modulation periods and show , that a significant percentage of stars showing long-term period variations could be classified as BL stars .

Conclusions :