## 1 Introduction  
In recent years, Augmented Reality (AR) has become a popular technology due to its integration in mobile apps (e.g., the Pokemon Go game [1]) and the increasing availability of powerful devices and development frameworks that enable the deployment of specialized solutions (e.g., for maintenance, training, or surveillance). In its most basic definition, augmented reality consists of combining digital information superimposed on a view of the real world [2]. However, this definition only emphasizes the visualization component, but sound interaction methods for object manipulation and task completion are also key for AR service delivery. In case of handheld devices, such as smartphones, the interaction is usually performed through touch control, while in Head-Mounted Display (HMD) alternatives, the interaction is in general performed through gestures, voice, or a combination of both or by using physical controllers. Actually, there is no dominant design yet for HMD interaction: although these devices enable more natural space exploration, real-world interaction metaphors cannot be directly used. Moreover, wearable AR (AR enabled through headsets/glasses as visualization tools) share the alignment challenge with mobile-enabled AR. This refers to the perception problems due to misalignment of the virtual content with respect to the real world, together with other issues that may affect the level of realism or the lack of precision for object/scene manipulation. In this paper, we explore the integration of haptic feedback as a way to enhance perception and manipulation of AR virtual assets. The contribution of the paper is threefold. First, we analyze how a 3D virtual object can be haptically represented to be perceived in shape and volume (aspects such as weight, texture, or temperature are not considered due to technology limitations), proposing some different representation options valid for different haptic approaches: mid-air, vibratile, or touchable. Next, we propose an architecture that is designed to facilitate the development of applications combining both AR and haptics (which we refer as _harptic_), on different types of visualization and interaction devices. This architecture proposes a data model and a workflow designed to be easily extensible and adaptable to new platforms as well as a series of artifacts ready to be integrated in future deployments. Overall, the main objective is to implement a natural interaction experience both when using mobile and wearable devices. Mobile devices can be integrated in the architecture either as visualization or haptic interactive devices. Moreover, the flexibility of the architecture is that the generation of haptic feedback can be adapted to any type of device. In addition to allowing the user to move in the real world while using them, mobile devices also enable to generate, e.g., vibratile haptic feedback, which may be integrated with AR experiences. For example, vibration may be used to show contact between the AR content and the device itself (e.g., the user is \"touching\" the 3D model with the device). Another use could be, for example, applying different vibration patterns to communicate a certain texture to the user. To demonstrate the feasibility of this architecture, we propose its validation with the construction of two proof-of-concept applications: (i) _Shape Inspector_, an application that generates 3D haptic shapes, with and without visual support for the user to identify them, and (ii) a _Simon-like game_[3], in which a system with four virtual coloured buttons is built and the user is asked to repeat a sequence of colors by virtually pressing them. Both prototypes combine HoloLens and mid-air haptics (i.e., hand-based haptic experience without contact surface enabled in this case by ultrasounds (Ultrahaptics device, UHDK5 version).