Feed

https://codex.wordpress.org/Customizing_Feeds

https://codex.wordpress.org/WordPress_Feeds

How to Add Custom Post Types to Your Main WordPress RSS Feed

functions.php

function myfeed_request($qv) {
	if (isset($qv['feed']) && !isset($qv['post_type']))
		$qv['post_type'] = array('post', 'story', 'books', 'movies');
	return $qv;
}
add_filter('request', 'myfeed_request');