Definitely not Oenothera biennis. Those grow in my garden (as an occasionally tolerated weed), and are strong upright growers with yellow flowers. My mystery plants are whispy (no strong upright stalk) with pure white flowers. I realize that my photo doesn't show the habit of the plants all that well (there's that dusk again), but these plants are no more than 6 inches tall, with no inclination to get any taller.
