function abo (heft, url) {
	var width = 960;
	var height = 600;
	window.open (url, 'abo_'+heft, 'width='+width+', height='+height+', left='+((screen.width-width)/2)+', top='+((screen.height-height)/2)+', scrollbars=yes, menubar=no, location=yes, resizable=yes');
}