Mapping one to (one or zero) help

Hello everybody,
I has tried googling, but I could find the answer. Could you please give me a help, many many thanks.
Here is my domain: one User mav have one, or no UserProfile
public class User : Entity
{

    public User() { }
    public virtual string Username { get; set; }
    public virtual string FullName { get; set; }
    public virtual string Password { get; set; }
    public virtual string Email { get; set; }
    public virtual UserStatus Status { get; set; }

    public virtual UserProfile UserProfile { get; set; }
}

public class UserProfile : Entity
{
    protected UserProfile() { }
    public UserProfile(User user)
    {
        User = user;
    }
    public virtual DateTime Birthday { get; set; }
    public virtual Gender Gender { get; set; }
    public virtual string Phone { get; set; }
    public virtual string PermanentAddress { get; set; }

    public virtual User User { get; protected set; }
 }