Have you tried:
var myVar = '123';
jQuery.ajax({
type: 'post',
url: ajaxurl,
dataType: 'json',
data: jQuery.param(jQuery('select[name^="option"]').val()) + jQuery.param(jQuery('input[name^="option"]').val()) + '&myvar=' + myVar,
success: function (mydata) {
// do something
}
});
I think you can also use traditional: true
instead of param