Hypnotic Siren

Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.

Wotc — April 26, 2014

Gaining control of a creature doesn't cause you to gain control of any Auras or Equipment attached to it.

Wotc — April 26, 2014

On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.

Wotc — April 26, 2014

Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.

Wotc — April 26, 2014

Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its {T} abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.

Wotc — April 26, 2014

If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.

Wotc — April 26, 2014

If you cast Hypnotic Siren for its bestow cost, and you gain control of a creature with a constellation ability, that ability will trigger when Hypnotic Siren enters the battlefield. You'll control that triggered ability.

Wotc — October 17, 2013