show latest post from multiple categories

im just trying to show a certain ammount of post from multiple categories in a page, i use always the next code to get post from a specific cat:

<?php $recent = new WP_Query(); ?>
<?php $recent->query('cat=11&showposts=9'); ?>
<?php while($recent->have_posts()) : $recent->the_post(); ?>
<?php endwhile; ?>

and i just cant figure out how to say that the category that should be display are , for example, 11,12 and 13

i tried width:
<?php $recent->query('cat=11,12,13&showposts=9'); ?>
but no luck

any idea?